My cat's just had a fit, four hours after I gave her Bob Martin's spot on treatment. (following instructions)
She's okay now, a bit dazed.
It's happened twice before, and both times we've taken her to the vet, who've not actually done anything, and have not asked about the usage of BM treatments.
I've googled "bad reaction to flea treatment" and its quite scary. I hope this helps other mumsnetters.