Our last date for Solar PV orders for installation in time to beat the next government update to the FiTs rates is 22nd February. Installations after this date will definitely only attract 21p/kWh, whereas orders placed before 22nd will possibly attract 43p/kWh, depending on the outcome of government legal action (which could very well be seen as just an action to try to discourage SolarPV installations ahead of the change)
After 1st April it appears that additional requirements will be put on installations accompanying other energy saving measures. See the Energy Saving Trust for more details.