Sale of flat that was originally bought with my late Mum

[Sideline]

Advise please on the level of cgt I am going to have to pay on this sale.
Details
Bought flat for £44k in 2004 one third me and two thirds Mum (usual buying costs say £1000 shared)
Property rented out since then
Mum died 29/9/2011 and flat then valued at £95k
As sole heir I inherit her two thirds of flat.
Have just accepted offer of £133k today (Capital costs of 4k to bring it up to sale standard plus 2k costs to cover sol/agent fees)
Std tax payer hoping that I only have to pay 18% on the original third up to Mum's death and then 18% on the £38k since then less all allowances and expenditure.

Is that the correct understanding or am I in for a bigger shock!
Thanks in advance for any help.

[maths]

The base cost to you will be the aggregate of your original contribution plus the probate value of the 2/3rds you inherited.

You are selling one asset (i.e. 100% of the property) not two assets.

No lettings or other relief available other than the annual exempt amount.