After a couple of weeks of sussing out why my car is only doing 50 miles to ten quid, I got to the root of the problem by cleaning the Coolant Temperature Sensor. This is removed from the block next to the manifold, it plug directly into the chamber where the coolant flows, checks the temperature and adjusts the fuelling appropriately. If the sensor is covered in a coppery residue (as mine was), it's not going to sense the temperature of the coolant too well, and your fuelling is going to be constantly high, because the sensor is thinking your engine is cold.
I used some wire wool on the metal sensor, and plugged it back in, and it's doing 100+ miles to 10 quid. Hoorah. Bought a new one direct from Vauxhall for 16 quid, and will probably get another improvement on that.
Another reason I'm getting really sh*tty economy is the idling. It's currently all over the show, when starting from cold it's up to 2000rpm, and fluctuates between 500rpm (which causes the car to cut out). The engine constantly runs at 1400rpm when warm. This eventually points to the components that are controlling or influencing the idling, the air filter (which can get clogged up) or the infamous Idle Control Valve. After about 30,000 miles the motor gets worn out, and can barely close the valve letting the air in, causing more air to enter the engine, hence high revs. I cleaned the sensor in the throttle body with quick sprays Carb Cleaner (from Halfords), this helped, but didn't erradicate the problem. The only solution was to replace the Valve completely, and the idling is back to a normal 900-1000rpm.