Thanks for all the advice and encouragement. I do have a tank sealing compound and am glad to have another thing to worry about.
Just want to nail this topic down with regards to my input.
Fitted a replacement head and had issues as discussed previously. Have refitted original and engine runs as before without missing or losing power.
To conclude i am confident the head and work to it were the problem and will start a new post on head/guide/spending at some point in the future.
On a lighter note, foibles (as RD says) is certainly a trait for these bikes. With all the on and off of head and rockerbox the carb has been much disturbed. Being of the this won't take long (?) mentality i tend to strap it casually to the frame to keep it out of the way rather than strip the cables etc. This has changed it's attitude to starting. I now need to engage the choke to start up. Had not used the choke from when i replaced the carb to cure the loose slipring (happy days indeed). Please post suggestions for causes in a separate space as i am wandering way off topic!
Right i'm off out to see if i still have that nice whistle coming from the rockers.
Big thanks to all the input and even more for the encouragement. The big hammer and grinder are currently being kept out of sight, mind you, lets see how todays ride out goes.