Found an interesting description from Hansard. Seems that the main problem is around jointing the pipework. So you may have problems if the fitter doing any work, was not familliar with working with steel.
http://hansard.millbanksystems.com/commons/1974/oct/30/gas-central-heating-west-midlands
I inherited a Servowarm central heating system installed with steel pipes, when I bought a bungalow in 1983. I was told that this company had done a massive nationwide sales campaign in the '70s. It was generally OK, steel pipes connected to steel radiators - no problem. The trouble is where...