In reply to Dr.S at work:
I'm not an accountant but do a lot of the basic book-keeping stuff where I work. My understanding is below, but worth checking with someone more specialist!
It is correct that you can't overspend the capex budget (from an accounting point of view), but it's fine to shift revenue into it. The difference is that you don't normally do this via a standard virement (ie just pick up the funds and plonk them where needed) - normal practice is to spend the money from revenue and gain it as income in the capital pot.
You create a revenue expenditure line called 'revenue contribution to capital' or 'direct revenue financing of capital purchases', vire the spare funds to that centre/line, and then spend the money against it. That is then entered into your capex cost centre as matching income.
You can do this as you go along as amendments to the budget, which can help to track things properly across the year. Or you can just do it at the end of the year to balance the cap headings as an accounting tidy-up before closing the books.