I've just finished ATPL exams with Bristol Ground School (can't recommend high enough). ATPL subject matter is harder than PPL, but only due to the additional complexity and greater depth that the subject matter covers. Its not that much more difficult really, but the amount of subject matter is fairly mindblowing and doing ATPL exams is a big commitment in terms of time - especially when distance learning and trying to hold down a job at the same time.
Its been a while, but from memory, PPL exams consisted of about a week of pretty hard core revision after work per subject. Multiply whatever you did for PPL by about a factor of 10 and thats about the amount of time you need to dedicate (rough estimate).
Then also you will need to do 2 sessions of compulsory classroom study with an approved training provider followed by a further week of exams, which equates to 2 lots of 3 consecutive weeks off work.
The best advice I had though was if you are thinking about it, get started ASAP. Its a fairly small amount of money in comparison to whats ahead, and you can start to study while you build up enough hours to do your PPL. You will quickly figure out whether you can cope with working and the amount of studing required before you have spent too much money.
Good luck - its fun really