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12.4V sounds a bit low for a decent battery. Slow cranking that speeds up is going to be either a dying starter solenoid or bad main power cables. Could be battery terminals, grounds, poor connection between the cable to the starter from the battery terminal any number of locations. But, unless it is cranking really slowly, it should still start.
Halfords don't actually stock anything beefy enough for a P38. 1000CCA is where it's at :)
https://www.batterymegastore.co.uk/hankook-mf31-1000.html
Sorry to hear of your troubles - I'm sure with a bit of TLC she'll serve you well.
Bad battery cables all day long.
Don't feel too sorry - he should know better :)