My Father purchased a property some 50 years ago and He gave…

My Father purchased a property some 50 years ago and He gave 50% of it to My Mother. Apparently the propery isn't registered with the land Registry. This apparently means they were JOINT TENANTS and if Mother or Father died the property would automatically pass to the surviving person.
In October 2003 Mum(77) and Dad(79) approached a solicitor to finalise and witness their wills. In a letter from the solicitor it states that the solicitor will need the property DEEDS to sever the Joint tenancy. In April 2004 Mum and Dad both finalised and had their wills witnessed by the said solicitor.
Father died 8 years ago and willed everything to His Next of Kin (2 sons, and 1 daughter) and also appointed them as executors. Will was witness and approved by the Solicitors.
Mother died on 13th sept 2016. Her will was also witnessed and approved by the solicitors, and appointed Her 2 sons and daughter as executors, stating that all assets should be divided equallybetween the 3 parties.
It appears that the solicitors never sent the deeds to the Land Registry between October 2003 and April 2004 to sever the joint tenancy and record a restriction on the Land registry.
The solicitors did however witness and finalise Mum and Dads wills in April 2004 that conflict with the property being owned as Joint Tenants.
Does this now cause a problem with the inheritance tax as at Dads death 50% of the property would have automatically passed to Mum by Survivorship. The house value is £650,000.
As I understand inheritance tax of 40% kicks in at £325,000. If at Dads death the property had passed to us as stated in His will then We would not be in this situation.
Can We now, still claim that it was Dads intention to pass His portion the property to Us on his death, and thus negate payment of inheritance tax.Regards
John Bantick