Hi Bill,
Like you I am mystified by the assertion by this lawyer. As you say the 1988 Act as amended by the 1995 regulations (which brought into UK law the EU Directive 93/98/EEC* of 29 October 1993) sets out the situation as you have described it.
When he says it's a bit more complicated than that, I wonder if he is referring to the situation where photographs have been made available to the public, under s.12(3)(b):
(b) if during that period [ie the period of 70 years from when the work was made] the work is made available to the public, at the end of the period of 70 years from the end of the calendar year in which it is first so made available,
which is clarified by s.12(5):
(5) For the purposes of subsection (3) making available to the public includes—
(a) in the case of a literary, dramatic or musical work—
(i) performance in public, or
(ii) communication to the public;
(b) in the case of an artistic work—
(i) exhibition in public,
(ii) a film including the work being shown in public, or
(iii) communication to the public;
The obvious point being that if the pictures have been exhibited to the public, then that becomes the start date for the 70 year period, not the date at which they were made. This situation is made somewhat muddier by s.104(5) which says:
104 Presumptions relevant to literary, dramatic, musical and artistic works.
(1) The following presumptions apply in proceedings brought by virtue of this Chapter with respect to a literary, dramatic, musical or artistic work.
....
(5) If the author of the work is dead or the identity of the author cannot be ascertained by reasonable inquiry, it shall be presumed, in the absence of evidence to the contrary—
(a) that the work is an original work, and
(b) that the plaintiff’s allegations as to what was the first publication of the work and as to the country of first publication are correct.
If the lawyer is not referring to the situation where the pictures have previously been made available to the public, then the only other factor to consider is recent caselaw. I am not aware of any decision on this specific aspect, so I can't help you there.
* The relevant part of the EU directive is Article 1 which says:
Article 1
Duration of authors' rights
1. The rights of an author of a literary or artistic work within the meaning of Article 2 of the Berne Convention shall run for the life of the author and for 70 years after his death, irrespective of the date when the work is lawfully made available to the public.
2. In the case of a work of joint authorship the term referred to in paragraph 1 shall be calculated from the death of the last surviving author.
3. In the case of anonymous or pseudonymous works, the term of protection shall run for seventy years after the work is lawfully made available to the public. However, when the pseudonym adopted by the author leaves no doubt as to his identity, or if the author discloses his identity during the period referred to in the first sentence, the term of protection applicable shall be that laid down in paragraph 1.
4. Where a Member State provides for particular provisions on copyright in respect of collective works or for a legal person to be designated as the rightholder, the term of protection shall be calculated according to the provisions of paragraph 3, except if the natural persons who have created the work as such are identified as such in the versions of the work which are made available to the public. This paragraph is without prejudice to the rights of identified authors whose identifiable contributions are included in such works, to which contributions paragraph 1 or 2 shall apply.
5. Where a work is published in volumes, parts, instalments, issues or episodes and the term of protection runs from the time when the work was lawfully made available to the public, the term of protection shall run for each such item separately.
6. In the case of works for which the term of protection is not calculated from the death of the author or authors and which have not been lawfully made available to the public within seventy years from their creation, the protection shall terminate.
Note that while point 3 refers to anonymous works which have been made available to the public, it doesn't refer to the situation where the work hasn't. However point 6 would appear to cover that case. There is no significant difference between the provisions of the EU Directive and s.12(3) of the CDPA 1988 (as amended).
Afternote. For the sake of clarity I should mention that the EU Directive 93/98/EEC of 29 October 1993 has been supeceded by Directive 2006/116/EC dated 12 December 2006. However Article 1 of both Directives is the same, and there are no other provisions of the 2006 Directive which directly affect this issue. 
