Stamp Duty Question

[nickd01]

Un-married (ex) partner and I own a house 50/50
She want's to buy me out - no worries.

Let's say house is £1m, mortgage balance is £500k so she has to pay me £250k to buy out 'my share' of the equity

How do we estimate the stamp duty payable? It seems to have the concept of 'consideration' - so is it based on £250k she pays me or on the whole £500k that my part of the property is worth?

I can't find a calculator that does the calculation for un-married partners selling to each other.

Thanks