Working from home and claiming expenses?

I needed some work done on my garden. We moved in about a year prior to having it done but 'lived with it' because we couldn't really afford it. Then a month before I started my business as a childminder we had the work done - out of neccessity. We only had it done because of the nature of my business (ie looking after children from home)

The garden was planks of wood (not proper decking) that had rotted through and had green slime on so was very slippy - ie dangerous. We had covered up the major holes by using big bits of plywood so not very practical or safe. We removed all the wood and removed the steps. We dropped down the higher part of the garden and  raised the lower part so there was not such a big drop between the levels. we replaced the steps with a slope for easy access for pushchairs and installed safety railings and low wooden benches running round the garden for children to sit on. It was concreted out so a flat surface for sand pit, waterplay and ride-on cars etc.

As the garden is something we would have liked to have had done eventually I am not sure if I can claim as an expense on my tax return? We certainly wouldn't have spent so much on it just for us. I would like to claim 25% as I use it at least 50% for my childminding as apposed to family use. (Childminding Mon- Fri - family use weekends only - if that!!)  The cost of additional materials and labour for the safety railings and childrens seating in itself equates to about 22% so I think 25% sounds fair especially with regard to usage but not sure. Even if someone could point me in the right direction for official HMRC advice I would appreciate it,

Thank You