I had this happen to me on my last open day two years ago, everything had been working fine then it just stopped. My receiver is under a 2 Ltr bottle cover on the stub branch of a tree, to give me the widest coverage of signal, on inspection, I found a family of Earwigs had moved into the blue box! Nothing I could do, after evicting said tenants, would make the receiver work. It went back to Massoth via 'Muns' , and the repair cost me £14 . It now works fine again, but I have sealed it inside the cover now. There was no explanation from Massoth as to what the problem was. It was a little annoying being away for several months but better than £100 for a new one, and I could still use the railway with the handsets tethered.
Dave