Check with your local CAB or council. Grants were available, certainly up to two years ago though you may have to pay part of the bill. Two years ago we had a complete system fitted complete with cavity wall insulation and new loft insulation for an up front payment of just under £800.
Google 'central heating grants for pensioners', lots of stuff comes up (not being flippant, but any grant would probably depend upon your MIL's satus which you probably dont want to share on here). The gov's Warm Front scheme ended on 19 January 2013 although your MIL might not have qualified.
https://www.freegreendeal.org
Had my loft insulation done on Saturday. Didn't pay a penny. The representative told me that in the to too distant future they will be doing free central heating boilers.
The only thing I can make out is that you need to either be a pensioner or on Working Tax Credits.