Paul:

There is an interesting two part article in QST May & June 1997
describing 300 W and 500 W output Class E (more efficient, non-linear)
amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
Since you are in the UK, there is a brief mention in Radcom (Technical
Topics) August 1997.

Ordinary garden-variety power FETs can also be used for HF linear
amplification using appropriate bias settings - though exactly how
linear they can be in practice I don't know.

Steve VE3SMA