An update.
Couldn't really figure if the leads are OK, sometimes some sparked, sometimes not. Got multimeter out and only 9 and 11 from leads/cylinders 5 and 3 and no reading at all from 1 (but sure I saw spark once but may be wrong).
So managed to get the DIS module out (what a PITA) as thinking it must not be sending to lead 1 (plus 2,4,6 needed removing plenum etc. anyway).
DIS looks relatively fine, however underneath the sockets for leads 1 and 4 at the end, I can see the plastic peeling away (about a 1mm gap) and I can a layer/sliver or two of dark rust. some small cracks on the top of the "end bit" too. Sorry no pics but the following image is somewhat worse than mine but I can see mine going that way -

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Does this mean it's on it's way out, irrespective of lead condition?
If so, I guess I need new DIS unit, re-fit and see if OK. If not, then new leads to be sure? Can I test the DIS with a multimeter? If so, how?
I see ECP sell the BOSCH and BERU at around £90 and ARC at about £50.
http://www.eurocarparts.com/ecp/c/Vauxhall_Omega_2.5_1999/p/Car-Parts/Engine-Parts/Ignition/Ignition-CoilsGiven I don't do much mileage (5k year) as have another car, will the cheap unit see the car out? I can handle replacing it if it goes in 25k miles/5 years down the line (doubt I'll still have the car then!!!).
Also, anyone know where I can get the HT leads (car is 2.5 petrol on a facelift model so individual leads from DIS to plug.) as ECP don't quote on their site. Vauxhall had some comedians in their parts dept, nigh on £200, I don't think so!!!
I'm nr Horsham if that helps or if anyone knows good local suppliers.