If anybody's interested, here's the story so far.
(1) Chappie bought unit, fitted it, and wanted me to arrange approval (aarrghhh, why don't people ask first).
(2) I come along, find the problem with flash rate - but so far as I can tell it does nothing else to cause a problem.
(3) Attempts to contact the strobe manufacturer prove.... that they've gone out of business (whether this is down to a failure to sell kit that has a chance of being approved I've no idea).
(4) I finally tracked down somebody at CAA who understood strobe approvals and we talked through it. It seems that the 40 fpm minimum in all the regs is to give a high-prob of even a fast "fighter pilot style" lookout sweep picking up the flash - the risk of an epileptic fit seem to occur at rates above 100fpm; hence the 40-100 range. So the conclusion is that we either
(a) bin it, or
(b) placard it as not a high-viz device.
So, we'll probably go for option (b) in day-VMC only, just so long as we can prove it doesn't endanger the aircraft or create unacceptable RF.
Certification's a great game - in theory designers will always ensure that what they are designing and fitting will be approvable. In practice far too often they design, build, fit, and then blame the certification engineer for making approving it difficult for them !
G