Hi Dan, the Omega's a lovely little gun, and regarded by many (including me) as the best springer Webley produced. (Gareth W-B is the board's Omega guru if you get stuck!)
I recently bought two Omega's myself , one's an 'as new' example that's tucked up in my gun safe, and the other's a 'very well used' example that I'm restoring for general use. I'm planning to send the action to Colin Molloy at Manchester Airguns for a re-blue (he quoted me £50 +P&P) - ISP airguns are also highly recommended.
Apparently, a lot of the parts for the Omega are interchangeable with the Vulcan Mk2, which gives another option if you can't find original Omega parts. For new parts, there's Chambers (www.gunspares.co.uk), and you can always keep an eye on the 'bay or on here for used parts.
Best of luck,
Dunk