Assuming the barrister's advice is correct then the gift by your mother to you has resulted in a GROB and the section 102B "let out" doesn't apply.
On your death prior to mum's death your estate will have an IHT charge on 50% of the property (as you own 50% in your own right) and in addition your mum's GROB continues on her part.
Her GROB can be terminated by her paying a market rent to you (broadly, 50% of market rent for the whole house).
So if im caught by grob being joint tenants if i was to sever the tenancy to pass my share on under my will to someone else, have i got around the grob by correcting it to tenancy in common. And does it start the 7 year pet again?
It's your mother who has made a gift which is then classified as a GROB on her part. If in lifetime you sever the joint tenancy converting it into a tenancy in common and on your death you leave your 50% to another person this does not remove the GROB on the part of your mother.
You haven't mentioned any figures and thus it is not possible to determine the extent of any IHT charges.