It's possible those old pipes have gone porous where you cut them James so even with compression fittings, they could be leaking round the olives. Only way of getting rid of the leaks is to have the gas that's in it recovered (that will also tell you how much you've lost), vac it down for at least half hour (preferably an hour) then refill the gas with the right amount of compressor oil, leak detector dye (usually a small charge for the dye) and leak sealer (again, usually a small charge) and errrrrr - oh yeah - 950g of R134a gas!
The new gas is R1234a "Solstice" gas and is about twice the price of the old R134a so they're trying to "price condition" people by jacking up the price of the R134a but what they can't condition people for is the fact the Solstice gas loses its effectiveness and tends to leak out after 2 years.
In other words, the sooner you do it, the cheaper it's going to be.