To be sure its the valves, and not the pistons/rings, the mechanic should have poured some oil into the cylinders to see if the compression then goes up. As a mechanic I assume he has already done that.

So, moving on, assuming the problem is with the valves, they really need to come out and be replaced or cleaned up, and that requires removal of the cylinder heads. Heads can be removed with engine in-situ, but as you say, it is cramped in the Tiger engine bay.

Bu ask yourself this question: has the engine ever been reconditioned? Has the clutch been replaced? If not I would consider doing a complete engine out strip down, and get everything checked. I speak from my own experience (see my ‘Clutch Replacement Blog’ for more info). My engine looked like it had never been reconditioned in over 40 years! I am now getting the benefit, and its a joy to drive 🙂 🙂