Have to agree with X-ray Alpha -
1 - coverage of 119.875 is poor below 3000 in general. Which is only to be expected in and around 3500' mountains. Not good below 3000' in the Oban area - you'd be better having a transmitter on Ben Nevis
2 - poor coverage from Loch Lomond to Stirling (where you get a constant clicking on 119.875 - this I believe is heterodyne interference which can affect GPS signals on some boxes)
3 - NATS Glasgow are excellent and very accomodating, they understand GA better than the airport owners. I operate freely in and out of Class D without a transponder (never causes a problem), and they are flexible enough to offer the short runway for expedition ..... which brings me onto another point .... in fact I'll open another thread for this ...