In reply to Pids:
As a a fellow long term mountain biker who has turned to the dark side (though I have not renounced my fat tyres), I cannot emphasise enough the importance of a decent pair of bib shorts. I went through the classic escalator:
1) Buy cheap (15 pounds) panel shorts (non-bib).
2) Realise these are rubbish for any ride longer than an hour. Buy slightly-less-cheap shorts, similar but with better padding (30 pounds)
3) Padding is much better, but the waist-line cuts into my belly after a couple of hours (note, I am not that fat). Buy basic bib-shorts (50 pounds).
4) Turns out that basic (i.e. cheap) bib-shorts still have a seam around the belly area. After 3-4 hours in the saddle, the slight tension in this seam really starts to press down on the belly! Buy good quality bib-shorts (80 pounds) with lowered 'ergonomic' seam.
5) Realise that shorts are only good for warm weather, buy equivalent quality tights and 3/4 lengths. Ask bank manager for loan etc.
In summary: you get what you pay for; but I would
strongly recommend you get a bib-short at least. And make sure they fit!