Thanks Plaiedes, firstly I apologise for not having renewed my subscription- I now have (hadn't realised it had lapsed).
It's worth every penny for the continued support.
Yes, I know the chain is slack, the bike is now 5 years old but I keep it cleaned and clean the chain with WD40 chain cleaner and use castrol racing chain wax, but have never adjusted chain other than mechanics changing tyres. I will look at that forthwith and publish my results.
I take it that with the same sprockets for the 5 years, I ought to replace the both with a new chain now (i don't stretch the chain with super rapid acceleration)?