I've used Plane Parking many times - in fact back in the early days before they had their own car park and used the Long Stay it used to be ridiculously cheap.
Still great value now, only problem is their bus stops are for show only and you have to walk through into the Long Stay to their stops. Still great value though.
Also if you book early enough with a decent promo code, you can park in the short term (surface) parking and walk straight into the terminal. £3.50 an hour short term but I've got it for under £40 a week. No lugging heavy cases on and off buses or standing around waiting.