I don't know about the SH stadiums, but in fairness Twickenham wasn't built just 16yrs ago. I wonder how much it would cost to install undersoil heating now. :chin: IIRC Murrayfield installed undersoil heating back in the 70s, so it must be possible.
From an article I have about stadium design.
MURRAYFIELD
Located in Edinburgh, Murrayfield is home to Scottish Rugby and was built in 1925. It used to hold the record for the largest ever attendance for a rugby union match, with 104,000 watching Scotland play Wales in 1975. At present, its all-seater capacity is 67,800.
It also plays host to music concerts including the final Live 8 concert, Edinburgh 50,000 - The Final Push, in July 2005.
Wintry conditions posed a constant threat to matches and so an investigation into the possibility of installing an under soil-heating system was undertaken in the early 1950s. An electric blanket was installed in 1959 at a cost of £10,000, but this system was upgraded by a new gas-heated system of hot water pipes. It was £250,000 to install and was ready in time for the 1991 World Cup. The new system has an estimated 24 miles of plastic piping running about 10 inches below the surface, from west to east.