Leggydan,
If you were only paying £40 per month for both gas and electric between you, this is quite low, hence why the final bill is so much more.
A good question is what is reasonable.
If she was using the washing machine and showering, (as you were) and you were using the heating and cooking where she wasn't, I think a reasonable compromise would be to split the total bill 66% / 34% (two thirds / one third).
HTH
Sparky.