Let to buy - lender wanting proof of funds

UKresident3333

My partner and I are currently in the process of turning my existing residential mortgage into a buy to let mortgage and purchasing a new house on a residential mortgage. 
We are on the verge of completing on our new house purchase. For this, we are receiving a 300k gift from my father which will allow us to put down a 40% deposit (this has been declared to our solicitor and the lender so everything has been done as it should there). 
Our issue is that the BTL lender who is going to lend so we can change our existing property to a BTL mortgage (this is a different lender to the one for our new home) is asking us endless questions that we have answered over the past 3 months and the latest one is asking for my father's statement showing the gift monies for our new property. I am not sure why they need this as we are about to complete on the new property so we clearly have the funds cleared to do this.
The main issue though is that this gift is from my dad's sale of a property a few years ago so the money have moved from different accounts over the years (e.g. he transferred some to my sister one year, then got some back and then moved it across his various accounts for different reasons). So if we send a statement, the lender may ask why the money is only recently transferred into this account.
Is the BTL lender asking for the statement because they simply want to see proof that we actually have funds for the 40% deposit for our new home (which would affect our affordability for the BTL mortgage), or is it a money laundering check to check source of funds? If it's the latter, how far back do they need statements as the trail would be endless with many different statements. Is there a quick and easy way we could prove these funds exist and their source? What do they actually want from us?

user1977

You send the statements for his other accounts too showing the funds being transferred between them. Not a big deal. If you want to know how far back, you’ll need to ask them - there isn’t a standard answer. Often only a few months.

jonnydeppiwish!

It’ll be annoying doing this twice or three times but do just provide them with what they want. They are the ones lending you the money so they are just ensuring that the funds have come from an appropriate source as they have to by law.

HampshireH

Didn't you have to provide this evidence for your new lender anyway? 

We had to declare a gift of £20k and they needed all this and more including pension and inheritance information for the person gifting.

(Why your BTL need it I don't know)

UKresident3333

HampshireH - yes, but our solicitor for the new lender is the same solicitor who dealt with my dad's sale of the property which he is providing our gift through. So proving the source of funds was very straightforward as it was the same solicitor who dealt with my dad receiving the funds from the sale in the first place! It's the new lender for the BTL that it's more difficult to provide evidence for 

housebuyer143

With let to buy the offer is likely valid based on your having a simultaneous onward purchase. They therefore want to ensure you have the means to buy this new place which involves you actually having the £300k you told them about.

I'm not convinced they are looking for money laundering as that's what the solicitor will do, more simply that you have the money available. 

UKresident3333

housebuyer143 said:

With let to buy the offer is likely valid based on your having a simultaneous onward purchase. They therefore want to ensure you have the means to buy this new place which involves you actually having the £300k you told them about.

I'm not convinced they are looking for money laundering as that's what the solicitor will do, more simply that you have the money available. 

Thank you - that is what I thought as well. Why would the BTL lender be doing the job of the residential mortgage solicitor?! Makes no sense