Three hours of plumbing in South London comes to roughly £165 to £210 in labour on hourly rates of £55 to £70, with a call-out charge on top if one applies. That is the straightforward answer. The question underneath it is harder.
Does three hours cost the same as four?
Widely repeated, and not established by anything you could check. No dated source publishes a UK plumber half-day rate, and none confirms that a three-hour booking is billed as a half day.
That is worth stating rather than papering over, because "three hours is charged as four" is the sort of claim that reads like trade knowledge and turns out to have come from an aggregator page with no method behind it. It may well be common practice among individual plumbers. It is not a published norm, and anyone presenting it as one is repeating something they have not verified.
What the published figures do support
The direction of travel is solid even though the half-day figure is not.
Longer bookings do attract a discount, because the plumber saves a second lot of travel, parking and dead time. Checkatrade's own hourly and daily figures imply roughly 13%, set out in the answer on plumbers' day rates.
That is worked out from the best-sourced figures available rather than asserted, and it carries a practical consequence for a three-hour job: it is always worth asking whether a half-day or day rate comes in under three times the hourly rate. Sometimes it will. Nobody can tell you in advance how often, which is exactly why it is a question rather than a rule.
The arithmetic for three hours
At £55 an hour, three hours is £165. At £70, it is £210. Against a South London day estimate of £400 to £450, three hours at the top of the hourly range is roughly half a day's money for three-eighths of a day's work, which is the gap the volume discount lives in.
Add the call-out. A booked, non-urgent visit is around £60 and a same-day or emergency one £110 to £180, all excluding VAT. Whether the first hour sits inside that charge varies by plumber and no authoritative source states a norm, so on a three-hour job the same work can be quoted as three hours plus a call-out, or two hours plus a call-out that includes the first.
Why the basis matters more at three hours than at one
At one hour, the call-out dominates and the hourly rate is almost noise. At a full day, the day rate dominates and the call-out is a rounding error. Three hours sits in the awkward middle where both are large, which is why two plumbers charging identical rates can quote materially different totals for the same afternoon.
The fix is to ask for a total rather than a rate. "What would you charge to do these three things in one visit" produces a comparable number; "what is your hourly rate" does not.
Making three hours worth booking
Three hours is enough for several small jobs, and the economics favour combining them. A dripping tap at £60 to £95 in London including VAT, a stiff stopcock, a slow cistern and a radiator that will not heat are four separate call-outs or one booking. Only the first carries the travel and the call-out.
Write the list before you ring, and mention all of it. A plumber who knows there are three hours of work coming can price the visit as a block rather than as a series of interruptions.
Nobody publishes the answer, which is why the question belongs in your first phone call.