We are getting our cellar tanked, probably with a waterproof membrane system.
The cellar conversion guy said that it was best to fit a sump and water pump under the floor to remove liquid water from behind the membranes, but it wasn't absolutely required.
If the pump isn't fitted, he won't guarantee the cellar against flooding.

The sump+pump adds about 60% to the cost of the tanking, so I'm a bit reluctant to do it if it's not needed. The cellar has never flooded as it is (but occasionally gets a trickle of water in on one wall when it rains hard).
So, my question is, anyone with experience of this sort of thing – is it worth spending the extra?
The cellar is going to be used for storage/laundry rather than a living space if that's important.

pfft I was lured in here by what I considered to be a wildly erotic topic title, only to find its just a load of underground bilge!

lol

We are having a cellar conversion done at the minute and we were told pretty much the same thing, however the guy did say he wouldn't recommend not having one, if anything more due to peace of mind.

Since he has started work he has found the water table about 1ft below the current floor level so its a good job we are having one. Our cellar space is approx 30sqm and the pump is costing approx £800.

Many people who quoted us said you had to pay them to come out every 1yr to a maintenance check at £200 a pop. Our builder says all they do is clean the filters etc and he will show me how to do it.

Just out of interest how is the pump adding 60% costs? We got 4 quotes and £22k was the cheapest, so the pump represents a minute amount of the total cost

Where abouts in the UK are you?

Well, the cost is a lot less than your 22k, as the cellar is smaller and it's not being turned into a living space, just a dry one.
£1200 on top of £2k for the membrane system.

Oh and I'm 150m up on a fairly steep hill, so no issues with the water table.