Can't you do it yourself? All you need is a set of spring clamps (halfords £10) and a socket set. That's all we needed for my old corsa. Took about an hour or so. Have to be careful the clamps don't slip though, or the spring could fly off and catch your face. That'll sting a bit!!!
I got mine done by Progrip in Bonnybridge, just along the road from Hypertech
Thing is with lowering springs you really should get the wheel alignment done at the same time. I found my rear wheels went quite a bit out after fitting the XPower springs. Progrip has the best 4 wheel alignment system in the central belt, if not Scotland
Tell him Ross with the ZS sent you and you might even get a discount