Hello Wayne,
I too would want to eliminate the battery as being the culprit, but to charge you £51 to check it is extortionate, and I certainly would be returning to them to check it again. If it were mine, I would take to a Halford’s store and ask them to test it for you. They usually do this FOC.
If it does test as below par, then it would generally be to your advantage not to buy one from there, but to come back to us for some recommendations of what brands to buy, and where to buy at competitive prices.
Kind regards,
Gareth.