So after much measuring, sketching, and cardboard engineering, with helpful info and advice from folks here and Andy at Woosh, I have worked out a reasonably practical way to mount the DP 20Ah battery, in spite of the very limited space. It isn't quite ideal, or as simple as just 2 or 3 screws into bottle mount holes, but I'm confident I can do it.
I did consider the bag battery, but felt that a solid mount was preferable, as were the bigger capacity and branded cells.
So this morning I ordered a Tongsheng 85TS kit with the 36V 20Ah downtube mounted battery, a throttle kit, and hydraulic brake sensors, all from Woosh for £569.
I'll take pictures and make notes as I go through the installation, and post here, maybe much to my embarrassment if it doesnt go to plan.