Never heard of this stuff before, but just found it on another forum. Great reviews and results endorsed by many owners. Dilution ratios can be critical: https://www.bilthamber.com/car-cleaning-and-degreasing/surfex-hd
Looks pretty good and worth a try! Might be overkill for you though Nigel seeing your bikes only gather dust! Feather duster should do it for you!
I tend to favour white spirit for light duty degreasing & comet for heavy duty jobs (comet leave a thin layer of rust protection behind) I hhave been using biltz concetrated shampoo for nearly 4 years on car & bikes and am generally happy with the results as it doesn't contain wax.