As the title says, my shrouds and lowers are fixed so I'm looking for a set of multisheave blocks to make them adjustable.

Many Thanks, John

Does anyone have anything suitable ?

Many Thanks, John

Hi John, you may have already done the maths, but the blocks needed are the high load variety. 
A basic shopping list might be 4 x triples and 4 x doubles for shrouds and lowers, add a couple of blocks for the jib adjustment and then some more for leading the controls back to the thwart. Looking in the purple marine catalogue, these will set you back close to £250-£300.  If you need 6 more cleats for the new control lines, you might add another £100.00
I may be wrong, but these blocks are probably only bought for a specific rigging, and are not usually the type of general purpose blocks hanging around in odds and ends boxes. 
Having said that, you might get lucky! I'd put my hope in e bay and search regularly!

Hi,

Yes indeed, I knew it wasnt going to be cheap. Just hoping someone had the shroud/lowers blocks in their shed (and wanted to part with them) before I went out and bought new.

Is 6:1 enough? The setups I've seen have 6:1 just below the decks and a further 2:1 under the kingpost.

Cheers, John

The boats with them fitted are probably still too young to be canabalised for spares. There is a recent topic to have a look at (link below) which might help - there seem to be many ways to skin a cat, or rig a merlin!
Its a big job to do from scratch.

I have a set of muscle blocks unused, you could have the two for £60.