After running normally, this morning my 97 Xantia had a hard time
starting, when it did eventually start, the revs quickly dropped and the
engine died.

After a few tries I could rev the engine and the revs would remain high
and constant for a few seconds, the injector warning light would come
on, and the revs quickly drop. Sometimes it will recover - the revs
raise again and the light goes off. Other times they will drop to idling
speed, the light remains on and putting a foot on the gas has no effect
whatsoever. After letting it idle for a while I can then rev it, and the
pattern continues,

The fuel filter is recent and clean - what should I do next?