Well, if it helps, here are some pointers on what NOT to do:
1) Assume it is like all the old Vauxhalls you've ever worked on and pitch up with a chain filter wrench and 3/4" socket.
2) Jack the car up, slide underneath and stare in amazement at obstacle number 1: a torx bolt on the sump.
3) Having acquired a torx bit and drained the oil, slide across to remove the old filter. 24mm socket required whereas biggest in toolbox is 22mm. Alley casting so probably can't get enough leverage with anything smaller than 24mm, eh?
4) Having now splashed out on a 24mm socket be amazed as the whole smegging housing starts to turn and various bits of dirt drop down into your eyes. As an added bonus, the seating is likely to be upset just enough for engine oil to start trickling down your forearm and up your sleeve.
5) Tighten the other way quickly and scratch head for a bit. Try loosening and tightening again. At this stage realise that although the housing is getting looser, it sure isn't coming off and the end cap is still securely welded on. More importantly, the housing won't tighten up either - car is going nowhere.
6) Following search of Omega Owners forum go out and buy a metal strap filter winch (£12.99 from Halfords but who else is open at 6pm on a Saturday night?)
7) Return to the car and slide back underneath. Now chucking down with rain and (not a lot of people know this) an interesting point is that the rain actually collects behind the bumper then somehow comes out under the engine and slowly trickles up your thigh towards your nads.

After a further hour of dicking about finally resort to mole-grips to get the band wrench tight enough to stop the outer housing rotating. Swore I was going to crack the casing at this stage. Hold the band wrench by jamming a bar across it onto the transmission housing and unscrew 24mm socket on end cap. End cap finally comes off.
9) Now thoroughly sodden through, blink through the tears, rain, oil and general detritus that's fallen in your face and try to work out how the smeg the housing is bolted on.
10) Another trip to the Omega Owners forum reveals that the housing is held in place by a 12mm hex key. Largest in toolbox is 10mm. Another new tool required. Begin to wonder if I've effed up and inadvertantly mixed up an oil filter change with a big end bearing replacement.
11) Screwfix only sell up to 10mm hex keys.
12) Same with B&Q.
13) Some Halfords sell them; some don't. Hope you get lucky.
14) 12mm hex socket, in the hole, tighten it up, we're heading home.
15) Reassemble with new filter and tighten to 15Nm. Not 50Nm. Not 500Nm. Not 15k Nm. In fact, looking at the rubber gasket I'm not even sure the cap needs screwing all the way until it stops. Certainly, when it does stop don't go any further.
16) Would you believe it? The smeggin rain has stopped.
17) Another interesting fact: Haynes have written a manual for the mig from 1999 onwards but don't appear to be releasing it. Has an ISBN number but the Haynes websitre says: under consideration.
18) Refill with oil and perform usual checks to ensure no leakage then drop the car back down, final fluid check and off to get dry.