I had the same accident on Christmas eve 2004. I didn't follow it up with the council for a number of reasons.
1) I rode over the diesel. If I hadn't allowed myself to become hypothermic while crossing the Pennines I might have seen and avoided it in time.
2) I sent all the witnesses away because I was OK enough to wheel the bike off the dual carriageway myself. I hadn't examined the extent of the damage to the bike at that point.
3) I was two weeks away from moving to the US then, and I thought I didn't need the hassle.
Regardless, I lost a lot of money because of the damage to the bike. And the broken wrist is never going to heal properly. My gear held up well, and I wouldn't have taken the helmet to the US with me anyway, so it could have been worse.
It's an interesting question though - do we sue people who are technically responsible, even if our common sense tells us that it wasn't really their fault, and there wasn't a lot they could have done to prevent it? I don't know the answer to that one.
I wouldn't blame anyone in my position (or yours) for trying to get some kind of recompense. Who knows, if it starts costing them money, maybe councils will start fining bus companies and hauliers over diesel spills? But I do think you need to make an honest assessment of whose fault you really think it was. Just in your own head, so that whatever you do you know it's "the right thing".
I tend to err on the side of taking ownership of whatever happens to me on a bike. As in point (1) above. I rode over the diesel, and being honest with myself I had to admit that it was a hazard I should have been expecting - on a roundabout on a busy bus and haulage route in the rain. If the slick was so ridiculously big that there was no way of avoiding it or staying upright, I'd probably have felt differently.
Anyway, after all that, I'm glad you're OK. Good luck with whatever you decide to do, and enjoy gear shopping. Just be honest with yourself before you embark on trying to apportion blame to someone other than yourself - if you're going to do it, do it for the right reasons...

EDIT: If you haven't already done so, visit
this site, sign the petition (if it's still running), sign the guest book, and tell them your story. Every little helps.