In reply to WJV0912: ..or even use a giant bouncy castle with lorry & generator.
I didn't realise my post would generate so many replies..or even controversy! Thanks for all the input. It's all helped. Plenty of ideas to try out.
I don't enjoy faffing around with all these contraptions. It detracts from the climbing. I'd much rather have a go at leading routes (it's got to be less faffy than this).
At the moment though I don't have a climbing partner or the balls/head to turn bouldering into soloing. About 4m is my limit for now. I'm pretty new to this game. What I do have is time during the week & a desire to use this time to get as much rock mileage as I can. So..for a while this is my current option.
I figure at least learning all this stuff will be useful in the future for belay setting & multipitch.
Anyway after playing I found the best way is to ab using the knotted rope, unknotting as I go. It's only in case remember..the idea I have is to climb up rather than slide down!