A let-to-buy mortgage can be complicated as it involves refinancing your existing property based on the potential rental yield, whilst simultaneously finding a new residential mortgage to fund the onward purchase of a new property.
Add to this the fact that buy-to-let mortgages are generally interest only, with rates usually being higher than on residential deals, together with the challenges of becoming a landlord and you will realise the importance of getting the correct advice and assistance.
There are merits to using the same lender for both parts but there are also potential benefits to using more than one lender and advice should be sought.