Sunday morning, and it's all back together and still working!
The body of the tender comes on and off quite easily - just 4 screws, though two of them are sunk deep into holes in the chassis and need a long-ish screwdriver to reach. Now that it's back on, the sound is SO much better than in last night's body-off first test, as the tender body provides a great sound chamber for the big Visaton FRS-7 speaker.
Despite Piko's recommendation otherwise, it is VERY much easier to do the decoder wiring with the tender body removed, as otherwise you're trying to fiddle with very short connecting wires within quite a narrow space - as Keith mentioned above, if only Piko would make the wires just a couple of cm longer it would be so much easier.... but in the end it all fitted, it's useful that, whether by chance or design, all the necessary connections to the Massoth XLS are at one end of the board - none of the terminals at the other end are used, which is good as they would be almost impossible to connect to without splicing and lengthening some of the wires! Presumably if you use the Piko decoder it screws back onto the four posts that originally held the basic analogue connection board, but this doesn't fit with the XLS so I trimmed a few mm off one of the posts and then secured the decoder to it with just a single mounting screw, so that it just rests on the other three posts, but its location - tucked right into the front corner when the body is put back - coupled with the shortness of the wires connected to it will stop it from moving around, I hope!
I would certainly commend Piko on making a loco where the decoder install - even of third-party kit like the XLS - is so relatively straightforward - a darn sight easier than most LGB installs I've attempted!
Off to a local G Scale Society open afternoon in a couple of hours, will take the BR24 along so she can get her first taste of the open (rail)road.....
Jon.