Lease/deeds, the words are used interchangeably in this case, but that answers my question, you have the lease.
It is the solicitors responsibility to check the property to see that the extent of it is what you are buying. What he would normally do (and should do) is send you a copy of the plan and the lease that goes with it and ask you to confirm that it reflects the area that you are buying the lease of.
If he didn’t do that then you have a claim to make to the Legal Ombudsman because the solicitors service has been substandard. You also have a claim to make against the solicitors insurance policy in negligence.
You have no claim to make against the selling agent because the document which is relevant is the contract and the agent’s particulars will be clear that they do not represent any kind of offer and should not be relied on and the buyer should rely on their own enquiries with regard to the property. It comes down fairly and squarely your solicitor to check everything surrounding the property and to report back to you accordingly.
You can not can rely on the words “garden flat” as any kind of description to say that the garden was included. It may be that the seller confirmed to your solicitor that the garden was included but did that incorrectly. That is the case then you have substantial claim to make against the seller.
I suggest that you see a solicitor to obtain the original file (not a copy) so that you can ascertain exactly what the solicitor looked at and what documentation he was provided with and what enquiries. Only then, will you know the way forward.
Can I clarify anything for you?