I know it is the wrong way round but my eldest is not very heavy and we want to reduce the travel easily and increase it as he grows. Is it doable? Do I just buy this? http://www.ebay.co.uk/itm/RockShox-Spring-Internals-Left-SoloAir-Sektor-26-27-5-29-Inches-150-mm-w-Top-/142343036035?

Doable, yes. Depends on if the inside of your forks stanchions have been protected from scoring by the spring, usually by a piece of acetate type material. If they have you just buy the air spring and fit. If not you’re buggered. Probably.

On-One Rev U-turn springs…

These

teethgrinder, that has got to be worth a shot, cheers.
I have found that my sektor fork tubes do not have the correct machining at the bottom to take that solo air kit I linked to up there.

teethgrinder – you appear to have solved my problem. Thank you very much for the pointer. I have offered up the new x-soft u turn spring assembly to the existing fork and it all fits. I just need to reassemble and test, but I now seem to have a reduced length fork with very soft spring. Thanks again.