Ask HN: Restrictive covenants for software developers?

I am wondering whether any HNers have any experience with restrictive covenants / restraints of trade for software development in the UK?

Situation 1: what's a reasonable time (if any) for an employer to block an employee developer from going off to work for someone else and help them develop a similar/the same product which may either a) compete with employers product or b) compete with employer's client's product

Situation 2: what kind of restriction can a client place on a software consultancy who's been hired to develop product X for them from then going off and developing the same product for another client?