Well here I am replying to my own request for information! After much fruitless searching I found a front paddock stand for full size bikes works a treat as a rear paddock stand for an XBR500.
£34.99 from J&S at Kimberly, Nottinghamshire. So if any 'side stand less' XBR/GB/SRX owners out there need a paddock stand you know what to buy and where. Alternatively wait until the autumn and buy one at Stafford, bound to be cheaper.
Incidentally, tried it on the front and with a bit of 'jiggery pockery' it works there as well, and as an added bonus it fits the frot end of my Triumph like a dream - hoorah!