:thumbs:Thanks for the comments Guys
The top up cans are a waste of money as Bobbeh has already mentioned.
The only correct way of re-gassing a system is to recover any refrigerant in the system, in Bobbeh's case he had 150g out of 750g remaining and then to draw a vacuum in the system (vacuum will extract any moisture from the system as well as provide some assurance against possible leaks), once the vac is done it is then possible to recharge the correct amount of refrigerant by weight.
Non of this is possible with a top up can, so don't waste your money, take it to a AC specialist.