Best deal for new mortgage

My brother is looking to purchase a council property with his parents and has asked me to look into some mortgage deals.

Here are the details
House price - £350k
Deposit - £75k (RTB) + £100k (deposit)

Had a look on Natwest, as he banks with them and this is their best deal according to the website:
2 year tracker - £250 cashback
35Years Term / Interest only / Tracker
Exclusive to: Customers who use us as their main bank - terms apply.
3.09%, 4%, 4.0% APR, LTV 60%, fee=£0, £450.62 / month

Is this the best deal out there, your thoughts and advice please...