Well Im glad to say technically it seems to have gone fairly well. This is the timeline according to Arqiva:
0031 - BBC2 analogue switched off from channel 62 at Selkirk tx.
0216 - High power multiplex PSB 1 switched on at Selkirk
0500 - Selkirk testing completed..
1005 - Eleven relay transmitters at Bonchester Bridge, Clovenfords, Eyemouth, Galashiels, Hawick, Innerleithen, Jedburgh, Lauder, Peebles, Stow and Yetholm were successfully broadcasting the PSB 1 digital multiplex.
I "just happened to have" a 10 quid Asda DTT box up in my office this morning

so decided to try it from my workplace. Galashiels relay is about a mile from work, line of sight. I didnt have a proper aerial so just stuck a 1m length of single core electrical wire into the aerial socket! Amazingly it actually worked well enough to get a full mux scan on the box. Although the pictures broke up badly it was good enough for a proof of concept.