The date of the above post is 28th October 2016,
Thank you SO much.
This was easy and worked very well.
I used a battery from Duracell Direct, which cost £50 (inc. delivery) which has a one year warranty.
I took the opportunity of cleaning the inside while it was open, using a soft sable-hair paintbrush, to lift the dust off the circuit board, and a microfibre cloth on the inside of the case.
It read 46% charge when first put in , (which may not be correct until it has calibrated itself) which dropped on use to 18%, and has recharged happily and just now it has reached 100% - brilliant!