Doesn't come up with a search on Merlins web site?
Amazon list various oil filters, various break in oils, engine oils and diff oils but no ATF that I could see.
However I have found these: AS PERFORMANCE ROYAL PURPLE TRANSMISSION OILS | ASP Trading UK
Ive been wondering about trying RP for quite a while.
Seems to do well in the opinion polls & the scientific breakdowns that I've occasionally read.
Nothing against any other brand and no allegiance to RP but think I'll use it on the Challenger and report back.