Thank you for clarifying that for me.
The area of law that deal with these types of cases for unmarried couples is the Trust of Land Act.
Your ex will have a beneficial interest in your property but only to the extent of his contribution.
Your ex could chose to apply to court under the Trust of Land Act for:
1) A declaration as to his interest in the property and
2) An order for sale for sale of the property so that his interest can be realised.
Please note that the burden is on him to prove his case and not you to do it for him. If your ex is claiming that he has extended the property then he is going to have to try and prove this. The information that you have been asked for seems somewhat excessive and I do think that they are on a 'fishing expedition' to try and get you to prove his case for him rather than him doing this himself.
You have to be aware that your children will be a consideration for the court - if he did make the application to court - and give that you have 3 children living with you in the property - with him having only contributed such a small amount - then the court would be highly unlikely to order that the property be sold even if the court did declare that he had an interest in the property.
What I would suggest that you do is respond and state your position that his only interest id the £4k deposit. Perhaps suggest a payment plan as to how this would be repaid to him.
I would also suggest that you consider referring your case to family mediation. There are lots of family mediation services and there will be one local to you. Just google family mediation in your area and give them a call to get the ball rolling. Family mediation will try and help you both agree his interest in the property and how this will be repaid without the need for court or solicitors. You can also suggest this to his solicitor.
If you are adamant that he has only ever paid the £4k, then hold your ground and do not worry. It is for him to prove that what he is saying is true. Whilst there would be no harm disclosing the documents that his solicitor has asked for - if you are adamant on him only having contributed the £4k, as your documents will corroborate this. If you cant get all the documents - do not worry - it sounds like he is trying to gather information to go to court anyway - so hold your ground about what his actual contributions were and ask his solicitor for him to provide you with the proof as to what he is saying about the extension. Tell them that you deny this and you expect him to prove it to you.
Do let me know if I can help you further.
Please kindly remember to star rate our service so that we receive credit for helping you today