We too saw the '50% off food' sign outside, and went in.
The guy on the door sold us the place (which was quite quiet at this point) on the basis of the offer. We ordered two different burgers (which were nice - especially my fancy one with triple-cooked chips and blue cheese sauce) and a couple of bottles of cider. Sadly we then went on to order puddings, which lay heavy on our stomachs - we wished we'd stopped at the first course as they were supermarket quality (actually, definitely not good enough for Waitrose!).
When the bill came - you guessed it, if you've read the earlier reviews - the food was marked at full price. However - unlike the earlier reviews - my husband just told them how the guy on the door had promised the 50% off - and they printed out a voucher, brought it to the table (where I filled it in) and then presented us with the correct bill.
So we're not complaining - for central London the bill was less than £30 for two course and two 500ml ciders each, plus the (already added) 12.5% service charge.
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