I have recently picked up a leaflet from B&Q which states that their supplier or subcontractor is able to install CWI to new build houses that already have a foam insulation barrier attched to the inside of the inner row of bricks or blocks. Is this possible?

My house was built in 2003 and has a 300mm outside cavity wall, constructed of brick outer, thermalite block inner with plasterboard attached via metal framework. Looking through various holes into the cavity, it appears that the inner solid poly foam is attached to the outsdie of the inner course of blocks, leaving a 10-15mm cavity. Could this be filled with injected foam?
If so, then this would improve the U-value of the walls and therefore reduce my heating bill.

Is this a 'do-able' job or would I encounter problems?

The house is detached, 2 1/2 storeys in height, buit by Persimmon Homes.

Thanks.
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energywise