Ok, no idea where I am going wrong here:
Not first-time buyer, purchase price is £400,000:
Stamp duty calculator says that stamp duty will be £19,500.
If I calculate following the guidance on gov.uk (www.gov.uk/stamp-duty-land-tax/residential-property-rates):
0% on the first £250,000
then 5% on the next £625,000 - in this case 5% on £150,000 (£400,000 minus the first £250,000 is £150,000) = £7,500
Feeling silly, but where is my calculation off?