What the Hell I Did. [Update]

polybikeuser Worst case scenario I'd have thought you had a pretty good case to take to the small claims court, you've handed a bike over to them so that they could remove a set of cranks with a crank puller, a job that should've taken ten minutes max and instead they've taken a whole lot longer angle grinding them off, in the process destroying the cranks and as a result overcharged you for the labour. So you have a demonstrable loss, the excess cost of the labour and the material value of the cranks, that you have sustained as a direct result of their professional negligence/incompetence.
Obviously pursuing a small claims action is both hassle and will cost you money, the costs are recoverble, unfortunately you just have to live with the hassle. An alternative way forward is to complain to trading standards who may well pursue the case on your behalf.

Poly - can I call you Poly, seems a bit informal? :) -

I agree with the sentiment of what you say here, but this is so cut and dried that there really is no need for recourse to the small claims court - ASM asked for a service (to remove his cranks) BLB agreed - but instead they destroyed his cranks - without wishing to sound glib = case closed.

BLB must (legally) meet its obligation here, to replace the destroyed item - the cost of the labour is a grey area I would rather not comment on.

As you say his loss is demonstrable. No contest here.

One other thing, (if it comes to it) pursuing a small claims action is super duper easy these days, simply log on to https://www.moneyclaim.gov.uk/csmco/index.jsp enter your claim + pay the online fee by debit/credit card (I forget how much but I think around a ton) which will be added onto your claim amount and that's it job done.

But I do really think that there is no need as this could be resolved much quicker by speaking to BLB (?).

polybikeuser There's no need to feel bad about feeling awkward complaining, you can almost bet that BLB and the like rely on this.

+1