Well, fingers crossed. Been under the bonnet for what seems like all afternoon. Turns out that whoever fitted the cam cover gaskets last time had been to B&Q for sealant, and then applied it librally to everything that they put back together, including the plenum bolt that they've stripped the thread from. There was red gunk (silicon sealant consistancy) on the underside of the breather box on top, around all the edges of the cam covers, on the EGR Valve, on the throttle body, and smeared on just about every pipe that I came across.
Anyway, gaskets done. Fingers crossed that when I turn the key tomorrow morning that everything fires up. Changed all the HT Leads as well, and not sure I've quite got the firing order right.
If you hear 'B0LL0X' at around 7.00am (no matter where you are in the country - I'll be yelling loud enough) then you can guess what's happened...
Hope your cam covers went more smoothly Lizzie! (I think there may have been a few done this weekend)