The subject of boiler testing is often raised in this NG (see Loco on ebay thread).
The two UK ME umbrella organisations (Northern Association and Southern Federation) both use the same set of regulation and these have been agreed by some insurers.
The above is an incomplete version of the draft but it is sufficiently near to the final document to serve for the time being. However, it contains at least one typo/mis-print . "... in the 500 -1100 bar life category ..." should read "... in the 500 -1100 bar litres category ..."
There is a PDF version of the final definitive document - somewhere..