Kevin, Have you tried the Giro Hex (I have a bountiful bonce and mine fits with room to spare!) or a Specialised Instinct (recommended when I last looked)?
CyclesUK.com were doing the Hex (RRP£50) for £35 incl. P&P when I got it about 6 weeks ago and I think they still are. A steal at that price; light, very comfortable, great venting, good coverage, excellent locking fit system and cool in the grey/blue colour!!
Giro Atmos for me (62cm-ish - I'm very clever you know). The "Roc Loc" system makes it fit, however weird your noggin's shaped (mine's long 'n' narrow). "Roc Loc" seems to be used on most of the decent Giro range, so I'm not just saying Atmos. I never wore a lid before - they all just wobbled about on top of my head or had about a hands-width of space on either side. Recommended!
A large Giro Havoc for me too, it was the only one of about 4 brands that I tried on in Halfords (Giro, Met, Bell and something else) that was big enough.
I thought I was the only one with this problem. I have a rather large noggin, measuring up at nearly 62cm. I was looking at the Giro Hex 2008. Anyone know if its true to it upto 63cm? Mine is quite a square bonce and I know some helmets are long and thin inside.