When I turn my oven on, it lights fine, but after a few secconds the gas used to flare up so that it reached the required temp. Now it just stays on barely plate warming heat, however long it is on for. Any views on what is amiss?
Most gas ovens operate by means of a heat-sensitive valve which only opens a little a first. This is enough to allow the oven to light. The heat from this flame then causes the valve to open fully.
Seems like something has gone wrong with this valve. You need to get it checked by properly qualified person
scotsman is correct, the flame failure device (ffd) has failed and needs replacing (it could be that the burner is blocked with gunk so its not heating the ffd probe but its usually the ffd at fault)
if your oven has an automatic cook facility we charge around �125 to repair, if it doesn't have auto cook facility its around �80 as the spare part is cheaper.