Here are the comments I made after doing just this job: (copied from my post of around Sept 2011)

The results



JB Weld arrived yesterday and was applied to the cylinder mid afternoon. I used a length of wooden dowel to transfer the liquid weld to the bottom of the cylinder, having first plugged the transfer port. I managed to avoid blobs catching the cylinder wall. Once a blob was at the base of the cylinder I smeared it around the base of the cylinder and then used a thin dowel to effectively push it into the cylinder / end block join. I was surprised at how much I had to use before I could convince myself I had filled the void ... I made up two quantities of weld so must have added about 1ml. I stopped when I could see a small rim of weld all around the circumference. Some weld was unavoidably left on the end block top.
This afternoon I sanded the exces weld away, using emery paper discs held onto a plastic rod with double sided sticky tape. The rod was just under the internal diameter of the cylinder. It took time .. a few slow rotations then removing the rod to brush off the powdered weld ... but eventually I had sanded down to the end of the block. I then added a thin rim of emery paper around the outside of the rod so as to remove any weld that may have stuck to the cylinder walls at the junction with the block. Worked down to a very fine emery grade. Total sanding time about 2 hours. Cleaned out cylinder and inspection with a light suggested all was well ... a nice ring of weld around the block / cylinder junction.
Re-assembled the rifle. First with the very weak spring I had used before. Immediate increase of about 70fps to give around 8 - 8.5 ftlbs (.177). Firtted a stronger spring and rifle is now a pretty consistent 11 ft lbs. Not too bad for a HW35.

So ... thank you all for the advice you gave. The effort was well worth it and the 'void issue' is one I will look for in future. I wonder if other makes of rifle can suffer the same problem?

Cheers, Phil