Thanks for all the good replies. I did check the shank diameter on that bit and it was ¼". It's a little-used spiral bit of good make. That collet I used was an insert that you drop into the ½" hole to bring it down to ¼" and the problem lay there probably. The confusing thing was that I tried several inserts, but the bit rode up in all of them. I did a search, ages ago now, for Triton metric and alternative sized collets, including writing to Triton without reply, and never found anything, but I should have looked again. The reply by Bstrom prompted me to search again on Ebay and yes, there is a place in the UK selling proper original Triton collets, including ¼" and I bought one, so now I am optimistic that this will solve the problem and I can throw out those inserts. Not only that, they also sell 6mm, 8mm and 12mm collets so that will really expand the choice of bits available. I did in fact try some 3/16" and ¼" bits with a half inch shank, but they just broke off at the base of the cutter after only a few minutes, perhaps due to my tendency to buy cheap garbage from Asia, i guess. Oak and maple was not to their liking. But - hopefully - problem solved and thanks so much for the help.