It was slightly before my time, but from bits and pieces I've put together: at one point Siân and Matthew were going to get hitched. It was all arranged, and Matthew was waiting for her at the altar. However, it transpired that Matthew and Llio had been having a bit of a fling on the side....and Siân had somehow found out. She went to the church in her white dress and everything, as though to go through the ceremony, but when she got to the altar she clocked Matthew a good one, and stormed out!
So, yes, Matthew, Llio and Siân have joint history. This has come up from time to time. e.g. when poor Nel was killed in a road accident (Siân's neice, Alwyn's daughter), Siân was understandably having a bad time, and Matthew was offering her comfort (no funny business, it was genuine). But when Sophie noticed this, she got all jealous, thinking that the Matthew-Siân relationship might rekindle. And on another occasion, Matthew and Llio took the schoolkids on a trip to France. We saw them coming back in the coach, and at one point, Llio laid her head on Matthew's shoulder and went to sleep. The look on Matthew's face implied that he'd quite like more of this sort of thing, so it was on the cards that that relationship might rekindle. But in the event, it didn't.
Well he owns Tŷ Pizza and therefore is a filthy capitalist living off the surplus value created by Jason and Kylie's labour (she still works there doesn't she? Although we haven't seen her there lately).
However, and it's never been made explicitly clear what he gets up to, but the suggestion is that he is really a wheeler-dealer, buyer and seller, who probably doesn't look too closely at the source of his cut-price goods which tend to be high-tech like phones and computers. (He had a stock of computers in the garage when the beer-brewers there managed to spill beer over them, at which he was not best pleased...).
So morally speaking, he's doing the same sort of thing that Arthur was recently trying to do, but he's probably a lot smarter and more successful. A little while back, Vince was trying to make extra cash, and agreed to deliver some fake pizza's to the address of what seemed like a neer-do-well. I think the pizza box contained phones, and the theory was that these were non-traceable throwaway phones for use in the drug trade.
He's always off somewhere, presumably to collect or deliver some of his trade goods, and is often on the phone to English speakers.
BTW, thanks for the google maps pointer!