uPVC exterior 'U' shaped sill - where do I get one?

[SenileGit]

I am hoping to build a porch outside my front door - there are already three three feet high walls onto which I want to put three windows, and then put a uPVC door in the space that is left (one of the walls is only a foot wide and there are steps leading down by it, which the door will go at the top of). I am trying to find a uPVC sill for the windows to sit on top of - I would have thought this was quite commonplace as my rear porch has a sill going around three walls, as does my neighbours, and I'm sure I read something about 'welding' the mitre joint of uPVC sills for this purpose - but I a) have no idea how to join two mitred pieces of uPVC (my sills and my neighbour's look perfect at the corners, as if they have been manufactured that way) and b) can't find anywhere online that sells ready made, mitred sills in a 'U' shape, i.e. with three pieces mitred together.

Does anybody know where in the UK I can buy such a ready made sill, or how I join mitred uPVC sills myself? I can't find anything on Youtube or Google. It seems like the entire methodology behind building a uPVC porch is a better kept secret than the Masons! Perhaps I need to learn the secret handshake and then all will be revealed.

 

[crank39]

You need to approach a window manufacturer and ask them to weld a cill for you, its not something you can weld yourself

 

[SenileGit]

Ah, I see - do you know of any websites that I can order one through? I have found several websites that sell all the other various parts I need to build the porch, including lengths of cill (is it cill or 'sill'?), but I haven't found any that offer the welding service.

 

[crank39]

Where are you getting your windows and door from, why not ask them to weld a cill for you, I'm confused why you insist on buying online, as I said before why not pop to a local window manufacturer and get them to weld one

 

[SenileGit]

I was going to buy the windows from somewhere like https://www.justdoorsuk.com, and I didn't think a window manufacturer would offer me just a cill.
Do you know of any sort of guide that exists to building porches - i.e. how the window frames fit to the cill, and so on? There isn't anything on Youtube about the very basics of fitting cills, as far as I can find. (There are several good videos about fitting uPVC doors, how to remove the old door and frame, and fit the new uPVC frame and door though.)