hydroscopic dot 5 fluid wear fast...ie absorbs water which decreases its boiling point...so needs replacing ofter. the stuff you on about is silicon brake flui, iei it wont ever absorb water, however, some people are not too fond of it cause it heat properties are different.
go you your work (halfords) and just buy the castrol super syntheic dot 4 brake flud, more than engouh, cheap, and just as good as the dot5.1 stuff.