That's the weird thing, since day one the other side of the wall has been a series of Electrical Installation bays and Hydraulic and Pneumatic workshops, but these aren't in use and nothing has changed in there whilst the fault has been there.
Interestingly, the SMART guide for electrical interference says it causes lines to be jagged, coming out like a heart monitor, rather than a constant wave form. Still, I'll give their guide a go and post back here if it works too