Thanks everyone for your time. Really kind of you to take the the trouble to share what you know. I've contacted the big cheese at the letting agents and he's onto it. I said that if it's not sorted this week, I'll take it to the authorities (as some of you advised). All the best and have a top weekend.
Inform your local water company. Under section 75 of the water industry act they can legally come in to the house and either repair the faulty float valve and bill the owner or turn off the water. Or if no one is in the house turn off the water at the outside stopcock yourself.
Here's the wording from the Water Regs: http://www.legislation.gov.uk/uksi/1999/1148/regulation/3/made Hope this helps - good luck!
You can't discharge waste water into rain water drains, ilegal. If this overflow is coming from wholesome water storage im not sure if this is classed as waste water on the other hand if this is coming from a feed and expantion cistern then it is contaminated water and this is not allowed to be discharged into a rain water drain. Hope this helps, ck