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Micklefield is a "Lite" transmitter. With your existing indoor aerial it will give you about half (the better half) of the freeview channels, including HD if your TV will cope. Micklefield is analogue only at the moment, so you will have to wait till switchover.
If you want the full range of services you will need to use Crystal Palace. Did you see 6 pairs of green bars against it on that Digital UK page ?
Your indoor aerial will almost certainly be inadequate for Crystal Palace so you will need to get one fitted on your roof. You will need a "Group A" hi-gain aerial.
Crystal Palace is broadcasting Freeview already (although at low power, so you may not get it yet).
Use the postcode checker's 'trade view' for your address to see if the Micklefield transmitter is the only suitable one (all greens after April) and if Crystal Palace is predicted to work for you then.
If I recall correctly the relay transmitter serves a valley area much of which stands very little chance of seeing towards Crystal Palace.
The room aerial you have that works well for one of the two relay transmitters, will almost certainly work well for Freeview after switchover (for the main PSB channels and the HD channels). Only if you really want the COM channels will it be worth spending money on a CP aerial. TV station by multiplex lists are here http://www.dtg.org.uk/industry/dtt_channels.html