Thanks Squidder for the great advice! I phoned around this morning as per the above recommendations. I found a Hobbyweld stockist (same company as Hobbybrew), they state that their gas isn't food grade, but have heard that it's fine to use. Mark One Hire in Rayleigh hold stock (they have another branch in Rochford that I assume does too). Spoke to a great guy £105 for 6 kg bottle and £35 for refill.
Was near the Groworks place in Southend so thought I'd pop in and see. £85 for 6kg bottle (and labelled food grade) and £30 for a refill.
I would have been happy to use Mark One but significantly cheaper at Groworks. Thanks again all.