If the solicitor concerned was acting for you in the purchase of the house then it was up to him/her to make sure the information on the Land Registry records was accurate.
If the property is owned as a joint tenancy then it would automatically go to the survivor on the death of the first of you, so would not need to be specified in a will. The alternative way of jointly owning property is as tenants in common, where a clause specifying this has to be on the deeds. If this is the case, then each party can leave their share to whoever they wish, so it would have to be covered in a will.
Without knowing exactly what the solicitor was referring to it is difficult to advise, but I would be inclined to get the situation checked by another solicitor to make sure your intentions are properly dealt with. You can get a copy of the land certificate from Land Registry gov uk (not one of the high cost imitation sites) for a few £ to take to a solicitor with your wills & the letter you are concerned about.