"then take another piece of lead which has also got a flat side , snap it so it sits just inside of the matchbox width-wise" he slightly contracts "which has" into "which's"

This is what I heard from the video, "then take another piece of lead, which has also got a flat side, snap it so it fits just inside of the matchbox, widthwise, then place it flat side down so it…"

You're right - he says 'fits' not 'sits'. He's got a midlands accent from the UK, probably from around the Nottingham area. He sounds like Paddy Considine in A Room For Romeo Brass. 'Which's' isn't possible in written English, not even in slang, but people do pronounce it like that when speaking.

To answer your comments in your reply to ian71... 1. You can have the contraction "which's" although it is not written that way very often. It is more a feature of speech. It is very common to string the two words together when speaking. 2. In the video he says 'fits'. 3. It is a British accent. Although, he does use a strange type of intonation which I have noticed on these types of youtube instructional videos. They all put on a weird voice - much like the artifical voice you sometimes hear in TV commercials.