it would seem to me that the easiest way to do it would be to levy a flat
rate tax on all developments that require planning permission and thus come
under PPG16, then, if the budget is insufficient for the year, the fund is
allowed to go "overdrawn" with the tax raised accordingly the next
financial year to compensate (bit like the council tax). In years where
there are a surplus, the tax could be lowered the next year. Tenders would
be sent directly to the curator, with them, not the developer, deciding who
gets the job, on the grounds of methodology/competance rather than cost.
Easy, I dare say, though hardly fair on homeowners. Also, penalising some
developments which do not threaten archaeology in order to pay for those
that do might be difficult to sell to the electorate. Although that's not
usually much of a problem for you average Chancellor of the Exchequer.