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This means that your partner and his ex-partner now hold their shares as tenants in common. It does not sounds as if the solicitor explained is clearly to you in his letter.
This means that the land registry will not register a transfer with only one person signing the transfer form, if held the interests were held as joint tenants and your partner died then the ex-partner could sell the property simply by signing the transfer form and producing a death certificate to the land registry.
The ex-partner could not do this now because the transfer would have to be signed by the executor of your partner's estate, it provides some protection against sale and essentially means the shares are independent held.
Your partner's share will now pass under the terms of his will (ie. to you) in the event of his death.
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