Apologies for missing your thread when it was first posted.
A couple of points on your proposed course of action.
A licence is a form of contract* and so it can't be imposed on another party without their agreement. If you follow that approach and go to the small money claims court (ie the local County Court, not the IPEC Small Claims track), there is every chance your claim will fail if the defendant denies that he ever entered into a contractual arrangement with you. Much the same can apply to sending a speculative invoice, unless it's backed up with something else.
In other words your initial contact with the alleged infringer should be on the basis that they have infringed your copyright, and that they face the threat of litigation unless they are prepared to settle by paying a fee to you. Once you have established a dialogue with the other party and they wish to pay your fee (or at least haggle with you about the amount) then you will be on course to establishing a contractual relationship. By way of an example, say a local window cleaner mistakenly cleans the windows of your house while you are out and then comes back later for his money. You can rightly say that you didn't ask for his services, and therefore you are not liable to pay him, although clearly if he's done a good job, you may well feel it is worth paying anyway. Of course in your case, the infringers are unlikely to be innocently or mistakenly taking your images, so you have the moral high ground (and the law of copyright) on your side when you go after them. However at that point there is no contractual relationship between you.
By the way, if you are unsure about what fees to charge, I suggest you base them on the NUJ freelance rates which you can find here
. Should you have to go to court, these rates will generally be accepted as the 'industry standard'. That doesn't mean that you couldn't charge more, say, for a particularly rare image, but since you don't normally sell you work, you may have more difficulty convincing the court that this is a fair rate. If any of the sites are serial offenders, then it is legitimate to add an uplift for flagrancy, but generally speaking the law would only expect to award damages to the extent that you would have received had there been a proper licence in place beforehand. Exceptionally if it is clear that the infringer has made a great deal of money by exploiting your copyright work without permission, then this could form the basis for assessing damages (known as an account of profits) if the matter went to court. However I suggest that by applying the relevant NUJ rate for the amount of use you believe the other site has made of your image (duration x number of page views) then you should come to a sensible figure.
My second suggestion is that you allow 14 days for the other party to respond to your initial correspondence and then if nothing is heard, follow it with a second letter/email with a further 14 day deadline. By doing this you will have complied with Practice Direction on Pre-Action Conduct
, which if it is not followed and you have to go to court, could result in less damages or costs being awarded. Note also that the PD also discusses, in outline, the content of any letter before action.
As for who you address the letter to, this can be a person or a company, provided the company is incorporated. Much will depend on the details you have managed to get from the website and Domain Name Registration (Whois) etc. Although I mentioned letter or email earlier, a letter through the post to a registered address is best if you have the relevant postal address. If you send a email make sure you get an automated message back acknowledging delivery. If you have a copy of any of the letters your solicitor sent on previous occasions, it would be sensible to base your letters/emails on his as he is likely to have covered most if not all of the issues in a businesslike way. Make sure you mark the letter 'Letter before Action' as again this will comply with the PD, and makes it clear that your initial contact is not a complaint form, which would indicate that you have commenced civil proceedings.
If you haven't already done so, read the Ministry of Justice Guide to the IPEC Small Claims Track
(pdf). It's pretty easy to understand, and if you should need to go down that route, you will be aware in advance of what hoops you need to jump through.
Although you say that the infringing party has a lot of wriggle room, in the most straightforward case they don't have much. Obviously they can deny infringement but with the procedure you have outlined it will quite hard to refute your claim. And if they don't put in a response to your claim, or fail to engage with the court or attend a hearing, you should get a default judgment in your favour. That of course does not necessarily mean you will automatically get your money, but it is relatively easy to get the court to enforce judgment, assuming the defendant has the means to pay.
Finally, you asked about how the IPEC Small Claims Court works. Like the county court small claims route, proceedings are relatively relaxed and designed for litigants in person. In many simple cases, there may be no need for a hearing, and the matter will be dealt with on paper. Case management conferences are often conducted by phone to spare parties having to travel to London where the Court usually sits. In theory the Court can move out of London but I am not aware if this has actually been done in the 18 months it has been in existence.
* Although I have said that a licence is not the appropriate way of obtaining payment in the present circumstances, a properly drafted licence
(pdf) (alternative here
) is enforcible because it will set out the terms on which you and the licensee have agreed to do business. There are sometimes terms which although they may not appear in the licence, may be implied, but that isn't particularly relevant to the present discussion.