You can get an external GPS device which connects to the iPhone via bluetooth. I'm tempted to get one for Destination Nurburgring as I don't want dodgy readings from lack of GPS! It's only £65 too.
http://www.ebay.co.uk/itm/2008044636...84.m1423.l2649
83SY what spec is your car? Can see a few bits where you just carry so much more speed than me, wish I'd had a bit of tuition now!
And to someone who said about having a few more seconds either side of Harry's Lap Timer videos you can, when you overlay the video you can add up to 5 seconds at the beginning or/and end