Update - it's going back!
I initially took the valves out and popped them back in again, which seemed to help. I discovered an odd feature - if the mid and bass control are turned right down, the signal becomes very weak - that faint fuzzy sound I described in an earlier post. This surprised me as I had read that the pedal was based on an old Marshall, where the mid and bass are essentially 'flat' when turned right down.
But I persevered as the tone with the knobs dialled up was really quite splendid. Sadly the pedal was plagued with an intermittent background static-type sound that was starting to give me the hump. I decided against opening it up and tinkering with it's innards and it's now being returned. Hey ho!