Technically, that would put you inside Stansted's airspace (1500 feet and above)! In reality it's 300 feet seperation.
However, dont forget jet circuit traffic at NW flies at 1000 feet AAL (1300 QNH) so your seperation form them is only 100 feet!
OK - I didn't want to put maximum of 399ft!
Didn't know jets are 1000ft AAL at NW though. And I've been there 2 years !