I have my doubts in that, seems to easy a diagnosis for said garage, were they busy?
I would get a second opinion, one that is well known or trusted.
Why not give these guys a call
www.lates600.com they are based in Coventry, you should get a more honest and accurate diagnosis.
If you were suffering piston slap since 11000 miles and gone away and come back, I would bet it isn't, it would get worse, not better. If at 11000 miles it was though to be piston slap, then the engine was not build to specification. Piston slap occurs in older engines where the bore is wearing and the piston's undersize, only the rings keeping it in compression.
When overboring an engine it is done in generally 20 thou steps, 0.020", 0.040" and 0.060", replacement pistons are also matched at the same increments. When boring an engine the new pistons are measured and these are what the jig borer uses to gauge the correct bore and tolerances. Not all sets of replacement pistons are machined to the engines build tolerances, they all vary a small amount.
A good case of piston slap is where someone has rebuilt a tired engine and fitted new rings and not measured the bores using telescopic bore gauges. It could be undersize thus requirung a rebore and a new set of pistons.
A sign of worn bores and a rebore required is when removing the pistons and seeing signs of scuffing on the skirts of the pistons, coupled with a step in the top of the bore jusdt below flush. But the proper way of doing this is to measure everything and seeing if the dimensions are within the tolerances of the engines blueprint.