Para 2 - am I correct to assume that the son's CGT lisbility would be based on the 2014 value of tge apartment (ie £242K) NOT the value at the time of gifting?
No. Son's base cost for CGT will be the market value of his share at the date of the gift to him.
Para 3 - I must admit, I don't really understand what you have written here? Could you give me a 'worked out" example of what the CGT charge would be?
Assume 242k when purchased by wife. Wife gifts 50% to you. No CGT charge on her part. You acquire the 50% at 121k.
You and wife then gift one half of each of your 50% interests to son when property worth 280k.
The gift by each of you to son gives rise to a CGT charge on each of you.
Wife gifts half of her 50%; her 50% was worth 121k when she bought it and now is worth 140k. Therefore her base cost is 60.5k of gift to son now worth 70k. She has a CGT charge based on gain of 9.5k (which is less than her exempt amount and so no actual CGT charge).
Same for you.
what scope, if any, is there to offset the purchase, legal, stamp duty costs etc against CGT when we both make the gift to our son?
Such costs can be offset suitably pro-rated.