Cash purchase,mortgage later.

Good evening,

I'm after a little bit of advice. I'm buying a house. The seller wants a quick sale and has given me a short deadline to complete(before christmas). Now...I've applied for a mortgage,got the AIP and started the ball rolling. Thing is, if the mortgage doesn't come through in time, i can still purchase the house with cash but have been told there is then a time limit on then re-mortgaging the house if i have to take this route. Is this right(told 6 months ownership) Must add the mortgage i'm after is a BTL.

Can anyone clarify if this is right.

Before anyone asks the cash has to paid back(not all mine) so i will need a mortgage

Thanks

Tunnel