Ocado Admit Error, But Won't Issue Refund!

So, my wife found a voucher through the door for 30% off our first 2 shops with Ocado. She decided to give them a try. After we placed our second shop I noticed that the total didn't seem quite right, we'd spent £139.71 (including all special offers) but only got £25 from the voucher. I then looked at our first order and saw we'd spent £93.94, but again only got £25 off. We should have gotten £41.93 and £28.18 respectively.

When I contacted Ocado via live chat the guy said that the maximum discount you can get with one of these vouchers is £25. When I told him that wasn't mentioned on the voucher he asked me to send a photo, which I did. He then went to speak to his manager and came back saying: "• Thanks for waiting, the reason for that is because the maximum reward is £25 it just unfortunately is missing from the leaflet. "
He did not offer to give me the money back initially. When I pressed him on it, he said that he'd give me a voucher for £20.11, but that I could only use it on a third shop. I explained that that is not fair and that I should receive the discount from the order we've just made. He refused to do this.

They have falsely advertised an offer resulting in overcharging the customer, and instead of issuing a refund they are saying: "we'll only give you the money back if you shop with us again". Is it just me or is that not ok? Is this a case for the MSE Money Savers? I also imagine it's quite widespread given how this voucher is from a canvassing campaign. I can post the chatlog if people are interested. Will post pictures of the voucher if I can work out how to do so on this forum!