?? loss of power is a bit vague? perhaps a few more details as to when and where and if anything preceded etc..
?£325 - for battery packs??
My old banger bike, (not in icon pic) can loose battery power when going over bumps in the rd as the old plastic mounting lugs have warped with sun heat and some have snapped off, a lil hdpe cut from a soap bottle as a wedge and a cable tie sorted me out 99% of the time
If batteries were ever plugged in while turned on?? the resulting sparks and arcs can damage the connecting plates and spring mechanisms - try testing with a plastic probe that your contacts are all sprung well and clean.