It is common practice for the contract to state that the tenant must have the flat professionally cleaned and have proof of this before you move out. You can check this in your lease.
Whether it was professionally cleaned when you moved in, is now unfortunately irrelevant, and should have been raised at the time if it was not up to standard.
150+vat is at the high end of the 1 bed cleaning scale I think, but you are unfortunately at the whim of whichever cleaning company the estate agents have paired up with to do this. No doubt the estate agent has added an arbitrary 'admin fee' to arrange this too.
The going rate for a 'domestic cleaner' is at the most £7 per hour in my area. Possibly a little more in London, but surely no more than £10 per hour.
You can hire a carpet shampooing machine from 'Homebase' for £20. (24 hours.) So, £150 plus vat to clean a one-bed flat seems exorbitant.
It's too easy to forget what a difference the vat element makes, now that it's set at 20 per cent. Your total bill for what I would guess to be a couple of hours work maximum, is £180. Ouch!
So if the contract was amended to say it should only be required to be domestically cleaned at the end of the tenancy and you did so, then you have a pretty good argument for them not to clean it again and incur no charge?
I'm assuming you cleaned it, but if not then yes they will have cleaned it and charge you whatever they wish, sadly. They probably own the cleaning company too :-P
If it helps I'm in the process of letting my flat and all the agents are recommending that a professional deep clean costing between 150 and 200 is carried out before rental, which the tenant is then obliged to carry out on leaving.
Why dont you ask for a copy of the specification, I worked for many years in the cleaning industry and always quoted for different levels of cleaning, for example, a basic clean would be what you would do yourself on a daily basis, then you would have a more intense clean that would include shampooing of carpets, cleaning of windows, washing of all walls woodwork etc, and I think Longlane is correct the agents have more then lightly added some on for their trouble.