The great advantage of adjustable ones is that they're often also demountable, so you can replace the valve in future without having to drain the cistern and make a mess everywhere - you just turn off the water, pull the valve off the stem, push the replacement on and turn the water back on again. Five minute job. Use Fluidmaster Pro45B valves - very reliable and has a brass shank so you won't strip the thread when you connect it up
I had been looking at FlushKING Complete Repair Pack 1 or 2, but you have suggested Fluidmaster Pro45B, which I will have a look at. I hope you don't mind but have you any experience of Flushking repair pack.
Many thanks for your reply.