Thanks all.
I don't use Sage.
I've found two solutions. One's Avalars's Excel add-on and is free. The other is VitalTax. £12 a year after the first year. They both appear to allow you just to choose which fields in a spreadsheet to send to HMRC. I'm going to look at both.
Progress, eh? I can't type into HMRC's system but if I type into a field in Excel and have software to send to HMRC, that's OK.
Vin