Hi Tiddles, it would come under Thames Water to resolve. Their number is 0800 316 9800. If you have problems with getting hold of them, please email me (details below) with the details of where it is and I will contact them on your behalf.
Cllr Renata Hamvas
Labour Councillor For Peckham Rye Ward
4th Thursday of each month (3rd in December), 6.30-7.30pm Nunhead Library, Gordon Rd SE15