Thanks for all the replies guys, some of them even made me smile ;o)
I lost the rag last week and nearly hammered their door down. Unsurprisingly they wouldn’t answer, so we went official: Well, it turns out the flat above is managed by the same company we have our flat through, and the property manager is quite sympathetic and doesn’t take any crap. First warning, and now [maybe to spite us] they have resorted to short blasts on the sound system combined with jumping. Last night they came in late (or early, depends how you look at it) and were loud enough to wake us up. Letter #2 will be going to the agency shortly.
And thanks for the constructive advice Rudeboy, honestly, very interesting!