My Windows clients connect happily to a PPTP VPN with RADIUS auth on the 3300V, but iPhone will only connect if local authentication is used. As soon as I set the 3300V back to use RADIUS, the iPhone fails authentication. I have tried 2 different RADIUS servers, one external and one on a server here, but the result is exactly the same.

Syslog on the 3300V shows the following:

pppd: peer 'john.smith' authenticate succeed, protocol:49699
MSCHAP-v2 peer authentication succeeded for john.smith
rcvd [LCP TermReq id=0x2 "Failed to authenticate ourselves to peer"]
LCP terminated by peer (Failed to aunthenticate ourselves to peer)
pptpd: EOF or bad error reading ctrl packet length
pptpd: cannot read packet header, exit
pptpd: read pptp packet failed

Can anyone give me an idea of what could be causing this or how to fix it?

I would try L2TP but there is no reference to it in the manual and nowhere I can find to put the secret in the 3300V's interface...

Your problem is MSCHAP-V2, if you knock it down to MSCHAP then it will work.

Can't help you on L2TP as that never seems to work for me either.