I have a combination boiler and today it has started constantly hissing even when I've turned the boiler off, I'm kind of concerned it could be a gas leak given it's happening even when turned off and I'm unable to contact the landlord until tomorrow, nor can I afford ringing the gas company and having a call out... what do :s

Well it was more, quick, ring whoever I think can help and in the meanwhile post somewhere I may be able to get help.