To followup on my own post - I've ordered a Cisco SPA112, since it looks like it might fit the bill, and just as importantly is available in a hurry!
In the last couple of weeks I purchased one of these units; surprisingly it doesn't come with the essential ring adapter! Bought quit a few ATAs in the past from different manufactures as these are given to our home workers to make business calls. My main gripe with all the ATA units is the default dial plan which is US based. The phone works more efficiently if it is replaced with a UK dial plan.
Over the years the cost of ATA hardware has risen quite significantly and with the added cost of the ring adapter and a corded CLI POTS phone, the total outlay is very close to an entry level VoIP phone such as the Cisco SPA301 IP Phone [for around £55]. VoIP phones make it a lot easier to use the extra functions that are standard with VoIP.
Worthy of consideration in the ATA market place is the SPA3102 because it has both FXS and FXO ports. This means that your connected POTS phone is abe to receive, and make, both VoIP and your fixed line service. But that setup leaves very little change from £100.