Took the Spitfire over to a tuning specialist today to hopefully sort out the carburettors once and for all. Expected a £300 bill to re-jet them and to tune them on the rolling road, but they told me that they were shot (no wonder we couldn't get them right!) and ended up with a £900 bill and two brand new carburettors. Even the old linkage was broken and had been 'bodged' by a previous owner.
Oh well.
I have yet to take her out apart from the drive home, but she seems to drive a lot better.