There was a fair bit of rusty scale sat in the bottom of the water spaces, but most of it appeared to be mineral based rather than flakes of rust.
It was a case of load the last bits in to the back of the car and off to fight the good fight with the steam cleaner. A couple of hours later and all are cleaned up and looking good. The inside walls of the water space look pretty sound with not to much sign of corrosion.
One thing that the steam cleaner bought to light was some figures on the crankcase.
I would imagine that is a part or casting number. It was hidden under the powdery paint and is usually behind the legs so you would see it anyway. I though I had better take a picture for the record as its soon to be painted over again (maybe someone else will find it in 65 years time when the engine is rebuilt again by another idiot!)
The next plan is to put a bit of protective paint on the legs and crankcase. Once that is done, I’ll re-assemble the crank and big ends ready to send off to the machine shop.