Yes Alex, I already found one that has the correct connections between the BT plug and RJ11, but it's quite long!
And thanks Tubaman I've found that eBay item but the shortest is 2 metres, similar to the cable I already found.
I do have one inelegant option as I have an ADSL filter (several actually) where the BT lead isn't captive, but plugs into the filter with an RJ11 plug. So I can just replace the filter with an RJ11 female-female connector. Smaller and less obtrusive than a filter.
But really Openreach should just supply an RJ11 faceplate.
Amazing what you find hidden away - that's a genuine BT master socket LJ2/1A.