When dealers talk about a % discount, the figure doesn't include VW finance contributions or loyalty offers etc.
A good price would therefore be between 10%-12% off list, then the £1250 finance contribution knocked off and, if you're eligible, the final £1000 loyalty bonus off.
Dealers only get 15% margin on a gti so they won't be discounting more than the figures above. As an example, Drive the Deal (who usually seem to be the best priced broker) were offering an 12% discount plus the £1250 deposit contribution.