Thanks everyone for your comments, especially Sally-Anne for such a knowledgeable and helpful reply.
A pleasure Freya.
Do ask around before choosing an EP. I have seen reports that are simply masterpieces and others that are disgracefully short - 2-3 pages, and clearly not written for the specific child. (Tell-tale signs like "Fred" suddenly becoming "John" on a later page.
Also ask which test batteries they propose using and check back with me. Although I am not an expert, I have read a fair few of these reports and I research the background information on each one in terms of the implications. Etienne is of course invaluable on such issues. I don't imagine she drops by "Everything Else" very often, but I will point her this way if her advice would be helpful at any time.
My DC very bright but have very similar SLDs - related to dyslexia but more complex!
Exactly - an EP report will give you a breakdown of the issues into their component parts, which is what you are missing at present. Things like working memory, short-term memory, processing speeds, visual ability, graphic ability, etc. Terms like "dyslexia" and "dyspraxia" are really catch-all phrases.
A report would also be very helpful in determining if any access arrangements should be made in future, especially for exams, although for GCSE a "refresher" report would probably be needed, because the evidence should be fairly recent. Do check that your EP isn't planning to retire or go off to do VSO in the Amazonian rainforest in the next few years!