In reply to nabeelfarid:
As mentioned there is a fair amount of sharp rock about be wary of going for a skinny rope.
Firstly there is Chert imbedded in the limestone, this is very similar to flint and can produce very sharp edges. Secondly some of the fossilised shells (which can be very prolific on some climbs) can be very sharp.
Elderid make good hard wearing ropes. Can't comment on that model. 10mm or there abouts would be sensible.
Unless you plan on climbing abroad anytime soon a 60 metre rope is a good length and covers all the climbing on Portland (as far as I know) whilst 50m does not.
Post edited at 16:51