I ordered straps for a friend's concertina from David J.Leese (Peter T's recommendation to you) and they arrived very quickly in the post.
Your Rochelle has more life in it even after you get a new one and would need the straps replacing before they break in two. Once you have new straps, you can find some help fitting them. At 2mm thickness, they may be a mm more than what you have. Just don't let who ever fits them over-tighten the strap screws into the wood. If they loosen and re-tighten the screws as they are now, they'll feel how tight the screw should be with the new straps.
The straps come with holes made but you may want to make another hole with a leather hole punch (lots of people have them, so you might not need to buy one). Exact strap tightness/ looseness, I'm still floundering with but what felt right for my concertina at the time, i achieved by securing the adjusting end of the strap (which had no nearby holes) with a long plastic cable tie and then marking the strap at the height of the hand rest and then punching the hole to maintaining the marked position. (one end of the looped cable-tie where the screw would go through the strap, the other at the other end of the hand-rest as low down and far away as possible). I was able to play my concertina with the cable-ties for a couple of days before deciding (bearing in mind the straps could come off, so not in a way the concertina could ever crash to the floor if they did).
From previous experience I discovered that if I was making a hole between two existing one's, it was better to appreciate how many millimetres of space I'm giving the strap rather than necessarily make it in the middle of the two existing holes. New straps will stretch over a year, so it's maybe worth noting how much daylight between hand and hand rest you started off with. Someone who enjoys very loose strap won't be bothered by the odd millimetre though.
I also bought "The Concertina Maintenance Manual" book by Dave Elliott for the same p&p which I'd not got round to buying before.
Eamonn C