Hiya eccles,
I feel the main challenge with the Mephisto is that, not only are most of the answers quite rare or obscure answers, but the cryptic make up of the wordplay usually plays with very rare smaller words and you find yourself guessing at what may fit in and then when discovering that a word exists, you seem to have to work backwards through it to try and identify the sections individually. My dad won the Mephisto about 20 years ago, and I remember him staying up all night to complete it. I had a stint of attempting it and sometimes finishing it about 12 years ago when I used to work nights and the job had plenty of 'sitting about time' , but had to make sure I had about 20 to 40 hours spare to dredge through the Chambers looking for pointers and rare word meanings.
However, The sense of achievement when you complete a Mephisto is second to none.
I'm not sure if he still does it, but the compiler, Tim Moorhey, does hold crossword seminars where you can pay a fee and go and listen to him talk about his crosswording (and maybe learn something new)
Keep at it if you have the time mate, as you will learn some really obscure words.
A good idea for anyone who hasn't attempted a Mephisto, is buy the Sunday Times and then buy the next one with the solution. Then look through the answers and see if you can actually identify his wordplay.
Hope that helps