The best place to get the filter/gaskets from is jpat
http://www.jpat.co.ukHow to check the fluid level :
http://www.omegaowners.com/forum/YaBB.pl?num=1152783586AFAIK ... the sequence is as follows (not done it yet .. but have got the bits .... )
With car as high as possible and as level as possible
Remove fill/level plug - ATF
WILL run out.
With a syphon tube/ suction tool/whatever, remove as much ATF as you can via the fill/level point
Undo the bolts on the large sump almost to the point of release, when ready remove all bolts but the 4 corners
Supporting the sump remove the last 4 bolts, the sump will be heavy and awkward as it is full of ATF. Dispose of old ATF
Remove the small sump in a similar fashion
The filter will be readily visible held on by just one screw, remove and either clean or replace.
Clean the faces of both the gearbox and both sumps of ALL traces of old gasket, this can be difficult and time consuming
Once clean replace both sumps using new gaskets.
Follow the procedure at the top to fill with new ATF.
You WILL spill ATF and it WILL drip on you as you do the filter and clean the sump mounting face on the gearbox.
It WILL stink and you WILL have trouble getting it out of your hair.
As I say, I've not yet done it, but this is the list I have built up having read other peoples reports over the past while.
If it is wrong hopefully someone who "knows" will correct it, and we'll both be able to do the job properly.
It is my intention to try and write a "how-to" with pictures when I do the job, my problem is getting the car level, as soon as I work out how to do that and get under it I shall be doing this job !!
HTH