I'm using one of these from Antiference
http://cpc.farnell.com/jsp/endecaSea...sp?SKU=AP00709
(add VAT and P&P if your order is less than £40 ex VAT)
It's diplexed in with one of their masthead amplifiers with 4 outputs so all 4 leads have DAB/TV available.
Be warned - I bought an earlier version of the same masthead amp/diplexer which filtered out the DAB frequencies as it was designed for FM/TV only. If you have seperate downleads it wouldn't be a problem but if you have a single downlead you may need a new diplexer/masthead amp/diplexing socket near your tuner.
Do you need one? If you get decent signal strenght on the stations you listen to then no.
If you're missing stations from what's available at your postcode then it might be worth a try esp. if you have easy access to your aerial as they're cheap if you buy from the right place.
Check your station list here...
http://www.digitalone.co.uk/coverage/search/default.asp