After fixing the broken latch, I had a look at the sill on the window to see what could be done to deter anyone wanting to get in that way again.
The solution was simple but effective, I installed a strip of wood along the sill to cover the small gap where the bottom of the window meets the frame when closed.
I repeated this on all of the windows overlooking the back yard. My client's fence is quite low and the place where the thieves gained entry is clearly visible both from neighbouring houses and the road, the only thing providing cover for a would-be burglar was a shrub in front of the window which I promptly cut down.
Home burglaries are crimes of opportunity and if we make it hard most thieves will move on in search of something quick and easy.
In my client's case, all she did was leave a shovel out, so it seems we can't be too careful when it comes to protecting ourselves and our property.