GoProPilot, I had the same dilemma a few years back, full time or distance learning ATPL, so I'll offer my two pence. (also considered FTA a few years back)
I did mine distance learning whilst holding down a full time job and I would do it again if I had to. Yes it was hard work and required a lot more self discipline than a full time course, however having that pay cheque come in every month and not be under any financial pressure was the key for me. I worked out when the brush ups were held, broke the exams down into 4 sittings to reduce the workload and set my own course schedule. I completed all the exams first time in 8 months with good pass scores and found that just breaking it down into those smaller sittings worked wonders. I used CATS in Luton (although back then they did brush ups at Gatwick as I was south London based) and I think I spent about £2500 in total.
I'd definitely say whatever route you choose, but especially with distance learning, use all available resources. I used the schools notes, BGS question bank and a couple of Oxford ATPL CD's for the trickier subjects (massive help). Some of the guys I met at CTC found it tough doing 2 sittings of 7 exams, especially trying to cram all the stuff in last minute just before exams and a fair number of them failed at least 1. I'm not saying that means anything, just feedback from those that did full time.
Difficulty-wise, I'd say it's more just sheer volume of information. In my opinion the hardest were Gen Nav, Air Law, Met and AGK (just for volume), easiest were IFR/VFR Comms, Human Fact, Ops Proc and Flight Planning. Ironically my best results were for the hardest ones.
Over those 8 months whilst I was still working, ok I spent £2500 on a distance learning course, but the 8 pay cheques I received whilst doing them meant I was about £25k up!
Hope this helps and best of luck.