I've packaged it up as discussed. The dimensions are 80cm long x 36cm wide x 38cm high, weight about 21Kg.
Checking out the FedEx website they didn't offer me Economy, the best was International Priority which costed out to about 200 GBP. Is it more effective for you to order collection and shipping from your end?
Royal Mail Parcelforce and UPS both costed out to about 155 GBP. Delivery time looked to be a week for either.
I guess I need to send a customs form with it to advise that it's being returned for repair?
Not hearing from Bruce with a shipping estimate from his end, I figured I couldn't establish a shipping cost myself until the amp was fully packed, so I proceeded with that.
Several layers of heavy cardboard, foam packing and parcel wrap later, it was fit to send (see picture). The cost was tolerable for an amp of that value, as long as it got there OK.
Figuring I didn't want to get stuck with import duty when the amp was shipped back to me, I checked with Bruce. His reply was initially of some concern - see part 26.