I've PM'd you Nac.
But generally, if you're approaching via the A1, leave at the Ram Jam ( a few miles north of Stamford, and signposted RAF Cottesmore).
Heading North off A1
If you want to be at the A1 end of the runway, after leaving the sliproad off the A1, turn right at the junction (away from red Cott signage). After 300 metres or so, at the roundabout, take the first exit and follow that road for a mile and a bit. You should see a load of cars parked up on the verge, adjacent to landing lights in field.
Heading South off A1
Travel a mile or so past the exit marked South Witham and The Blue Cow Inn. Then leave at the Ram Jam exit (fuel station and Ram Jam Inn visible on your right, on other side of carriageway). At the bottom of the sliproad, take the 3rd exit off the roundabout, and head under A1. Immediately after that, you'll come to another roundabout. Ignore the first exit (red signage Cott), and take the second exit, and you'll be then on the 1.5 mile stretch of road as above.
The threshold and viewing area that I used the other day is just a 2 minute walk across a field (there's a stile and public footpath from the verge). You'll see the spotters. If you want to be at the other end, thats slightly more complicated. For ease of access, the A1 end is far simpler if you don't know the area. And better too, especially if she lifts at that end.