Thanks SB. British Gypsum tech dept didn't take that view. They don't have a test to confirm the set-up so they are telling me that even though I have an existing 60, I need anther 90 below, that is if I want corroborating data. 
Two possible answers, depending upon the question...
1) Is it acceptably safe as an installation where 90 mins fire resistance is the recommended level of protection? I'd personally not have a problem with it, but it's a qualitative argument relying upon precedents for other installations (along the lines mentioned by Seabass);
2) Could you demonstrate that it has the relevant level of fire performance recommended? No, you couldn't, & if the configuration were actually fire tested it may or may not give 90 mins. A particularly pertinent fact is that the furnace exposure between 60 and 90 mins (furnace temperature between 950
oC and just over 1000
oC) involves far higher heat fluxes and energy transfer than between 0 and 30 mins (ambient to 850
oC). The fire protection has to work much harder in the last 30 mins of a 90 min test than it does in the first 30 mins, so 30 + 60 most definitely does
not reliably give 90 mins, in terms that could be demonstrated.
So... BG are absolutely right in what they say. If this relates to building works, and is subject to Building Control inspection then I'd not take the risk, unless I'd cleared it in writing with them beforehand. They're becoming very picky about such things these days.
