Hi Steve
When I first got my TDi I was also getting low-50s. A combination of a few mods, a few bits of maintenance, plus an evolving driving style, has seen my average MPG go up fairly significantly.
Low-50s is actually pretty reasonable if you think about it, there aren't that many cars that can churn that sort of average MPG out without too much effort. However, if you want to chase higher MPGs, you could consider the following:
1. change the thermostat and temp sender - even if they're not showing as particularly off, there could be an underlying pathology here - the A2 is rather renowned for the stats and temp senders going technical, and they're cruicial for maintaning optimum MPG.
2. consider fitting the 1.2TDi aero mods - wheel arch lip spoilers, sub-frame spoiler and exhaust tunnel cover. It improves the aero a little and might give you a bit of an MPG boost.
3. increase your tyre pressures - I used to run my 15s at 42PSI. MikeMars runs his 15s at 46PSI. Higher tyre pressures reduces rolling resistance and improves fuel economy. Try it - it's free!
4. Switch off air-con until engine is up to 90.
5. Switch off the engine at traffic lights/jams (if you think you'll be stopped for more than 20secs).
6. Splurge big bucks on a transmission mod to lengthen gearing - either a longer 5th gear (which is what I've done) or adding in an extra 6th gear (which is what MikeMars and Timmus have both done).
7. Be really obsessive about eeking out more miles from modifying your driving style - eg. experiment with pulse&glide, working out where you can use overrun fuel-cut off mode, lighter acceleration, change up earlier etc...