Hi Jim, and welcome.
Omega 3.0L MV6 is a very nice car indeed... and yours is a 2000 with only 50k miles

As for the cambelt, I had mine done at a Vauxhall dealer for £320, so the price is about right, though you should be able to haggle it down.
It is not a terribly complicated job, and any good garage can do this provided they had done this before and have the correct tools (locking kit).
It is also not beyond the competent DIY mechanic, many people on this forum have done it themselves and even produced a DVD which is available to purchase for the modest sum of £3.
A few important notes about the cambelt:
a. It’s not the actual belt that fails, but it’s tensioner. Therefore both the cambelt AND tensioner must be replaced – they would usually come as a kit anyway.
b. The bad news is that if the tensioner fails, it will cause serious engine damage, including 16 to 20 bent valves and possibly scarred pistons, which means a new engine unless you intend to rebuilt it yourself (the parts themselves are expensive enough, but if you add garage labour costs a rebuilt would be prohibitive).
c. The even worst news is that Vauxhall quietly reduced the replacement interval from 80k/8y to 40k/4y because the tensioners were failing early, however there have been reports here of tensioners failing even as early as 36k! Your car is 10k and 2 Years over the limit - I would seriously suggest that you do not drive the car until you get the cambelt kit done.
d. One last point is that the proper locking kit is absolutely essential – it can be hired for £10-£15, or someone here might even lend you one – but don’t embark on the job unless you have it, and definitely don’t entrust the job to any garage that doesn’t have it! The timing will never be right without the locking kit.
Once you sort out the cambelt, seat back and enjoy the ride… it’s a fabulous motor!