Thanks for the replies.
Despite attempts to persuade me otherwise, I admit that I had an inkling that these trial flights were illegal.
My intention was to find out how normal / widespread / acceptable that this practice was. I am still of the opinion that this sort of thing must happen at alot more schools than appears to be the case on here so far.
My first objection to the situation is a financial one. Our instructors are losing out on pay and flight time. Trial flights are paid for in advance and if there is no legitimate instructor available then they should be postponed, as they do due to the british weather. If these flights are carried out by the salaried desk staff, the school saves £15 per flight. If an instructor is told not to come in to work then they save an extra £10 per day.
Whilst most trial flights are a birthday present for little jonny, they are Exercise 3 of the training syllabus and must therefore be carried out by an instructor. This bit is clear cut.
As for new member check outs, PPL currency checks, differences training, aerobatics training and type conversions e.t.c. This is probably more of a grey area. I admit that I am not certain of the exact regulations and it appears from other posts that it is wholly dependent on the club in question.
I have a feeling that club rules and pilot order books everywhere have received little updating over the years and might need reviewing to make sure they conform with current regulations.
I have of course raised the issue of these flights at the school and have been met with responses from complete apathy to "I didn't know they weren't an instructor". I will push the issue further with the school (at a bit of a career risk !). I will not be pursuing the matter with anyone outside of the school. If there is no satisfactory conclusion, I will just walk away from the organisation with my feelings well known to all at the school.
Thanks again for your inputs and I really do hope this situation isn't as widespread as I first thought. I do not want to attract negative attention to the industry or the attention of the Belgrano (though a memo arriving on the door mat of all flying schools clarifying the situation and outlining consequences for non-compliance might be welcome !).