Defender - what size spare wheel cover?

[ajprice]

What size spare wheel cover would this Defender need for the wheel on the back? No idea what the tyre size is but its an LR alloy with off road tyres, picture attached (apologies for shakycam blurriness!). It's Christmas present buying time, and to ask for wheel or tyre sizes or going to their house and looking at the tyres would raise suspicions just a little bit. Looking for the soft vinyl type of cover, not the hard plastic case. Any brand I should look for, and how much is too much for a vinyl cover?

Thanks