I see that The Cat Group are recommending neutering at around 4 months:

http://www.thecatgroup.org.uk/ (Got to Policy Statements, number 1).

And that the GCCF endorse this:

Welcome to the Governing Council of the Cat Fancy

Also, The Cat Group have a paper concerning anaesthesia of kittens for neutering (link is in the policy statement mentioned above).

The two Cat Group papers are quite technical, contain proper academic references, and may help to convince a vet who perhaps doesn't specialise in feline matters.