What are peoples' opinions on flat panel heaters? I saw a largish one
in Wickes but it was only 900W and didn't say how it worked (oil, wire
element, etc). I also saw a smaller DeLonghi one in Homebase that was
oil-based but that was just 600W.

Surely these can't heat a room as well as a conventional oil or
convector heater (which can be 2-3 times more powerful), so is it worth
getting one?

I'm happy to settle for a free-standing 7 or 9 fin oil heater (1.5KW)
but the flat panel heaters seemed more discreet.