Sacha wrote:

I can find no
other nursery than Burncoose that has even heard of R. Jackman's Prostrate,
other than one which no longer lists it.

I think it is fair to assume that any plant sporting the Jackmans
name, must be an old or fairly old English variety due to the
association with Jackmans of Woking. That said, it is virtually
unknown apart from the plants offered by Burncoose and therefore its
true identity must be highly suspect. There's a lot of difference
between a plant originally acquired from Jackmans (that they may have
acquired from another nursery/customer/friend etc.) and a variety
raised by Jackmans and given their name. The chances are it is
probably a recognised variety (in S. Europe) and that the correct name
has been forgotten or lost along the way. As a result it may have
spent a short time as 'prostrate rosemary from Jackmans' and it
doesn't take a great leap for it to become Rosemarinus 'Jackman's
Prostrate'. It happens quite a lot in the trade resulting in one
plant having several names. I wouldn't be at all surprised if Olivier
declares it to be 'Barcelona'.