Well as an update, I haven't got hold of any silicon or stove rope yet but I did bend out the Weber lid lip with pliers.
The fit isn't perfect but its a massive improvement.
I had my first proper smoke on it at the weekend. 3kg of boneless shoulder for pulled pork (I couldn't get bone in).
After lighting, it took me about 1.5 hours to get the temp where I wanted it (225F). I put the pork in, kept watching it for an hour, then went to bed (nervously!).
I was amazed to see five hours later that the temp hadn't moved. The pork took 10 hours in total and I also cooked some ribs.
All turned out very well indeed. I was very happy with it.
I let the UDS keep going to see how long it would stay at temp and it started to dip below 23 hours later....!
I knew these things were efficient but that is just outstanding! For reference, I used Big K restaurant lump, fist sized chunks and nearly filled my basket (probably about 5 kg).