You do end up knowing your car intimately if you do it yourself though.....stripping all the interior out to get to the blinking thing. Why on earth they didn't run the cable under the car I have no idea! The £300 was a rough estimate from Mike Duncan whose rates are very reasonable......hate to think how much the factory would charge!
Luckily I did not have to change the cable in mine, just the bolt in the handbrake itself, took a good while to get the seats out, rear and front tunnel etc.
But I agree there must be an easier way - perhaps in a pipe underneath?