Technically you would have to pay CGT as it isn't your principle private residence.
However if your mum is going to keep the entire proceeds of the sale, then maybe you can gift your share to your mum prior to the sale and the she can sell tax free as it is her principle residence. You might want to get some advice, but I think it might fly.
the only proviso is that you have to survive 7 years. It worked for Cameron mum.