What the hell can they be doing that takes 3 months?

With the school holidays the impact will not be too severe for the first six weeks but will be a nightmare after that.
The lower end of Park road, between Asda and Harris lane really needs to have double yellow lines the whole length, on the Glaxo side, to make the road fit for two way traffic again. The majority of the parking along there would appear to be GSK contractors anyway.