So we did a little investigation and found that the washer does have two pipes leading from it to our flat. One for cold and one for hot water(I think). Then we used it and compared the meter readings. It's definitely using our boiler to heat the water! We're getting someone from British gas to check it out and give us something in witting in case I need to take this to court.
Until you get it sorted, you can partially control the use of your gas. Might be a pain, but turn off your boiler at the wall switch or turn it off by the off button on your boiler. That will prevent your gas from being used. The weather is warming up, so you shouldn't need to have your boiler on as much -- just turn it on when you need it.
Another thing you will need to look at is water. If you are paying a water bill and the cold feed is coming from your flat, then I suspect you are paying for the water as well.
If the washing machine is communal and there for everyone's use, then your landlord should have a separate supply for gas, electricity and water to power that machine. Unless he can show you these communal supplies (and their associated meters), I'd be very concerned that he's powering a communal washing machine off individual tenants' supplies.