I was browsing on here last night and came across the tip to use a ratchet strap over the seat and round the swingarm to simulate the weight of the rider when adjusting the rear chain.
So this morning I taped a tall piece of cardboard to the chainguard, climbed onto the rear ot the seat and used the mudguard stay as a reference to mark the height of the seat with me on the bike. Strapped a big ratchet trap round the seat and swing arm and compressed to the marker.
Blimey, the rear chain was tight!
No wonder it felt like the suspension was bottoming when I hot a bump at speed, but more significantly I may have solved a problem I've had ever since I built the bike. I have had to keep re-adjusting the primary (belt) drive as it keeps tightening up. Well, now I have and explanation for it. The tight rear chain must have been pulling the gearbox back slightly
I will take for a hard run in the next few days to test this out.
Even if this is not the issue I must express my thanks for the tip on loading the suspension with a ratchet strap. Sorry I cannot recall who posted it but I owe you one. Its tips like this that make this forum invaluable to me.