My 2005 RRS is really bad on mpg around town, I live in Milton Keynes so it's all dual carriageway roundabout so stop start and always short journeys 2-7miles so never really gets warm so I know it's the worse kind of journeys I do but I'm getting 15mpg this is driving very sensible too I dread to think what it would do if I booted it!!
On motorway @70mph it's doing 30mpg

Is this right or could there be a issue with it?

First of all, do NOT rely on the trip computer, recheck the fuel via brim to brim. In Spain - 2018 F Pace - Frightened away from the Sports due to crank failures
Gone - A couple of RRS, A couple of Disco's. However thinking of another Sport.

15 is low, my round town, dual carrigeway stop starts give me 18-20. Motorway 30. I got the garage to clean my map or maf sensor, not sure which one. It was groggy and that seemed to help a bit. I'm not Fat. RRS TDV6 HST.

On mixed short journeys I am getting consistent 22 mpg (brim to brim) during Winter. The computer shows 25 mpg. Average speed on computer shows 25mph. Nothing over about 60mph as the only bit of dual carriageway I use has just been reduced to 50mph and 30mph round roundabouts (Blaby by-pass).