Witch Hazel - I've had a look at this. The vouchers can be bought in all sorts of values, but the minimum is £10 to get the 1p off the litre and the example they give elsewhere is that if you have a voucher for £1.35 off a litre of fuel and the fuel only costs £1.32 then you don't get change. Here is the example cut and pasted from their website.
Before you shop at any of these retailers come to us first and save on fuel. For every £10 gift card you buy at Morrisons, we'll instantly give you a Fuel Saver coupon worth 1p off every litre of fuel next time you fill up with us. For example, buy a £50 gift card for the latest looks from Debenhams and get 5p off a litre of fuel.
The question is, are you going to get value from the Vouchers. In my case, if you can get Amazon Vouchers, which are not mentioned on the website, and if you are buying something big from there anyway, then it is a good idea. If it is just for groceries then I am not so convinced. A lot of the deals that I would want are not as good as Tesco. There are plenty of vouchers, like Debenhams above, or Boots, etc, but watch out for the Visa as there is a fee on the use of it, which is more than we would save unless we topped it up a lot.
I think if you are going to Morrisons anyway, and if you usually top up at a Morrisons garage and it's not out of the way, and you are going to spend that amount in a store that has these vouchers and you are going to top up with petrol/diesel in the next 63 days then it is worth it.
There are pitfalls. This is no good if you have to drive a distance to the petrol station. The fuel you use could cost more than the saving. It is no good if you impulse buy more than you save when you call in the store (a real problem for me), or have to drive a distance or get bus fares. It is no good unless you the voucher sensibly for stuff you need. The big thing for me is - would using the voucher still cost more than shopping around for a better deal somewhere else? After all, eBay is my friend and I've had some good deals from Tesco recently. Neither of those have vouchers at Morrisons.
DH does top up at a Morrisons petrol station regularly and gets around 30 litres each time, around once a month or so. If I got an Amazon card (not listed on the website but I saw them in store), and put £100 on it, then we would save £3 on the petrol. I could load the voucher on to our account and look for the best deals on groceries. I would have to make sure that the deals I got on Amazon would be either equal or better than Tesco, so that I didn't end up spending the £3 I saved and more on the groceries.
It's better running this round my mind than worrying about all the other stuff that's taking up headspace.