Thanks for your questions
Lets look at how things are, and whether there is scope to better them.
Your mothers house, will always have a capital gain implication for the year that it has stood empty, as things currently stand, due to the fact that you have a main residence during this period, in which you remained resident. But if you then take up residence there, then this will only be a concern, when and if you come to sell in the future, when that 12 months period will need taking into account.
Your current main residence, has (until now) been your main residence throughout, and so you have the time you lived there, plus a further 36 months after you move out, that will be covered under the private residence relief rules, so even if you have not yet sold your main residence, then there will only be a capital gain implication should it take more than the 36 months, from the date you leave this property. And this 36 months exemption applies, even if you let the property out.
But if you decide to keep it, then after this 36 months, then a capital gain position arises, which may be reduced if you decide to let it out to tenants, as a further relief called private lettings relief is due, which can allow up to a maximum of £40,000 exemption on any arising capital gain, and of course you have the annual exemption allowance - which currently allows a further £10,600 exemption in the year any gain arises.
If, however you chose to keep this current main residence, for weekend use, then just the private residence relief is considered, so if you hold onto this property beyond the 36 months, then a capital gain will arise.
If you let the current main residence, then this will require you to declare rental income to HMRC.
Now to look at whether this position can be bettered.
You do have the option to elect that your mothers home be treated as the main residence from the date of purchase, and as long as this election is made within 2 years of the purchase of your mothers home (so by Nov 2013) and this would mean that you clear yourself of any capital gain arising on your mothers property, as you plan to move here, however, this will really only be of benefit IF you sell your current main residence within 3 years from Nov 2011 (so by Nov 2014) because by making this election the current residence ceases to be physically the main residence from Nov 2011 ( as you can only have one elected main residence at the time)
And of course will have no benefit it you plan to keep your current main residence , for weekend visits (as then its just swings and roundabouts as there will still be a capital gain of the same position arising, its just a question of which property this additional year sits with)
So its really a case of reviewing the position nearer to the time that you make the move, and establishing your intentions at that time (as to whether there has been any interest in selling the property, or whether you plan to let the property out)
But if you need more advise at that time, with your actual intentions, you can always come back to Just Answer, and ask for me Sam Tax, if you would prefer (just ask for Sam Tax in your opening sentence)
If you have any follow up question, then please do ask, but if you ensure that you rate the level of service you have received, it would be appreciated, as this ensures I am paid for time and expertise. And any feedback and bonus are always welcomed.