What else can you get for £800
Even if you spend 2 grand on a car
Doesn't mean it will be perfect better the devil you know
Unless you know of little demons arising .
Unless the tickover is so erratic that's its almost stalling it could make the light flicker but unless you get it running constant
Its going to be hard to establish pressures so tick over and as said
GL
Correct oil and level ?must be level cold car
If ok wire to oil pressure switch loose from vibrations or corroded
Oil pressure switch
Oil pump pressure test
Crank bearings
You choose
In that order :)