In reply to Darkskys:
At that price from Go Outdoors, I'd snap it up, and QUICKLY before they sell out, assuming you are intending to just us it in Summer conditions. It is stupidly cheap - in fact I can't get 10mm rope that cheap at trade price even by bulk buying it 1000metres at a time, direct from manufacturers!
However, I'm not sure if you noticed but it is a sale item and only available at that price at one store - Thurrock! So you probably end up needing to look elsewhere.
For Winter use, you would definitely want a dry treated rope, but for Summer use it is useful but by no means essential. 10mm is a decent thickness for a 50m rope but if you to look at a 60m rope, I'd certainly suggest getting a thinner and lighter one in the range 9.4mm-9.8mm.