Perfect, John! Thanks a lot.
And as it happens, that green background actually looks quite good (I realise you only put it in there as a temporary clarity measure so I could easily follow the alignment change to the box). Not sure I won't stick with it - although it sorta clashes with the purple header banner, I guess.
And thanks for the link to the validator. Ronpat made me aware of this facility in a previous post, but I haven't had time yet to study the errors and fix them.
One question, if I may. It seems most of the errors relate to the html within the Wordpress page editor. No line numbers show up in there, and although I can readily identify the problem lines via the quoted phrases in the validator, I can't always see the html code that needs fixing. Do you have any advice on how to get beneath the bonnet, so to speak? I've asked this question before on the WP Support forums and not received a clear answer. Maybe it's too basic for tech-savvy folk to bother with!
Since I've exposed myself as less than well-educated in code matters, I've got nothing to lose by asking another question that has been perturbing me. When using the W3 CSS validator, line numbers are given that also do not appear in the WP css stylesheet. How is this best dealt with, do you think? Would I copy and paste the WP stylesheet code contents to some other editor that does display line numbers and do the corrections there, then copy and paste the code back to the WP stylesheet once corrected? If so, do you have a recommendation for an appropriate editor that does display line numbers? Someone recommended Notepad, but I've never managed to figure out how to get numbers displaying on that.
Much appreciate your help.