Can I stop my neighbour closing in a porch way. House Martins have been building their nest under it for over 20 years & return every year. I'm worried that if the work takes place the birds will disappear. They generally have 2 broods so 8-10 babi

You can only object to a neighbour enclosing a porch if it contravenes any planning regulations, that includes planning applications not applied for if required.

However, some extensions etc can be exempted if they've been up for a minimum period, even if planning consent was required.

Many extensions etc are exempt if they are less than a specified area of coverage, that you would need to check with the local council.