"Tax doesn't have to be taxing! (but we'll do our utmost to make bloody sure it is)"
I had much sympathy for Revenue staff when a few years ago they were lumbered with new software which, I mused, might have been written in the Far East because it appeared to have no regard for Western accounting processes. They got my tax code right in the end - at the third attempt - when I held their hand and helped them disentangle their rats' nest. I was able to do this just because I had kept meticulous records - yes, that is so important.
That sympathy has now evaporated in the heat of searing evidence of wanton job-creation which has just hit my desk; hmm, I guess they are facing cuts as well ... the mind runs to kukris.
Tips:
Yes, it is a very good idea to do your own sums. It is not hard if you have the right information, and don't just rely on the self-assessment bumf which is something of a mushroom job.
Be wary of HMRC "help" sheets, some of which are poorly written and may be expensively misleading. The taxman's internal guidelines are now readily accessible on the HMRC website; however, in a case of ambiguity it can pay to go the extra mile, right back to the wording of 'The Act', again easily available online.
Although the pulp-fiction tax books are not really up to much, I have in the past found Tolley's Tax Guide to be very helpful (the serious grips and dodges for grown-ups, in fairly plain English). It is somewhat expensive; may be worth checking your library service for a recent edition.