There is a problem here in that you are still married and the house you live in was the matrimonial home. It is the current value that matters not what the house was worth at the time of separation. Likewise it matters not one jot that he transferred the house to you and signed something at the conveyancer's. The conveyancing lawyer should have pointed out to you that such a document had limited value while you were still married and if not you should ask why not.
There were two ways in which the agreement you reached could have been made formal and final.
1. You could have entered into a formal separation agreement. This isn't very common because it more or less binds you to obtaining a divorce based on separation which can delay a divorce for up to five years. A separation agreement would have needed to have dealt with all financial claims arising from the marriage so it is a rather more comprehensive document than just transferring equity.
2. More commonly you could have settled the financial issues formally and finally within the context of a divorce by way of a court order.
You did neither so all claims are still alive on both sides. That is the bad news.
The possible good news is that he may not have much of a claim. I say may because it is impossible to know for sure. There just isn't enough detail to know. The fact is that he has bought a property of his own and you also live in a property which is in your sole name. To that extent your respective needs are satisfied. It is unlikely that a court would think it reasonable that you should sell the property you live in just so as to pay him a lump sum for which he has no obvious need. It is not decided in the sort of arithmetical way you are approaching it.
That is not to say that he might not be entitled to some lump sum. It is impossible to know on these facts. For instance, if you earned twice as much as he did, there was a relatively low mortgage on the property you live in and you could easily remortgage to raise a lump sum then a court might find that reasonable. On the other hand if you already have a substantial mortgage, earn much the same as he does and would struggle to raise a further lump sum from a remortgage then it would probably be unreasonable to expect you to do so in order to pay him a lump sum for which he has no obvious need.
Whether he can successfully claim more depends more upon factors like this. One thing is certain which is that if this settles by you paying him some more then it is important that you only do that within the context of a court order (which can be obtained by consent) in divorce proceedings. If you do it in a less formal way - as in the way the equity in the house was transferred - you would run the risk of finding yourself in a similar situation again perhaps years from now. Any further payment must be done within the context of a formal and final settlement properly drawn up in the divorce.