Range from the cheapest replacement of a toilet float valve £50 (Plumbforce Direct Ltd based in W1U) to the dearest replacement of a float valve £90 (The Kensington Plumbing and Heating Specialist based in SW7).
Comparing plumbers estimates for replacing a toilet float valve in the cistern tank gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes to replace a toilet float valve in Marleybone by posting a job with HaMuch and we will send it to plumbers in Marleybone.

"Toilet in basement no longer refills, so won't flush. I believe the issue to be an internal fill valve issue or blockage. The pipe goes behind drywall so hard to identify the exact fault. Just looking for quote at this stage. "
W1S
" Replace the float valve in the toilet cistern. I have a classic closed-coupled toilet with a cistern behind the toilet. The know gets stuck in the cistern, and as a result, the cistern is not flushing. I think that the mechanism is rusty and might need to be replaced. When can you do the job and..."
WC1H