Ok so researching braid choices further..
I have already have Gardner Kinetic braid (normally used for carp and catfish) IIRC and this works (I've used it on a couple of trips) and fills 1/2 the reel spool (including having mono backing).
Looking on the rainforest - there's lots of Chinese braid that reviews indicate that when tested it breaks below the quoted rating and abrades especially when seawater comes into the equation.. so looking through the UK sea angling sites.
Spiderwire Stealth has good reviews for seawater use and a spool gives 300 yards (274m) at 50lb. I'll put a weak link in to stop loosing the braid if it snags the weight or lure.
I'll move the Gardner over to replace the braid on the little reel on the 20b rod - the line on that came with it is faded and I don't know the breaking strain or how much is on there. Although the braid will be 50lb, the leader will still have a weak link in.
Edit: a bit more... about the leader etc so the configurations will be:
50lb rod - 50lb Spiderwire Stealth (moss green, 274m) + 50lb shock leader (enough to attach boom) then 40lb Ultima New Sea Fishing Enigma Snood Mono tracer on french boom, 15lb mono from 60lb shock leader onto weight as a weak link.
20lb rod - 50lb Gardner Kinetic + 60lb shock leader and then 15lb mono tracer for hooks / weight.
There's an interesting comment here about thicker line (i.e. >=30lb) to lures making the lure wiggle rather than the tracer in the tide:
http://www.fishingdartmouth.co.uk/wreck_pollack.htm