Hi,
I haven't done this, but from talking to people who have.
CBI rules are that at the time of purchase, they will need a valuation of what the house will be valued at after renovation, and that's the value the bank can lend against. So if you're doing structural works, you'll need an architect to draw up plans before you buy. The bank might require QS costs for proposed renovation.
It's not too difficult to get the approval, or to get the initial mortgage to purchase the property.
The funds for renovations will be drawn in stage payments in a similar way to a self build. The bank will likely insist you have an architect and a contractor to complete the work, and you'll need sign off at different stages to draw down the funds.
I've heard it can be a little difficult to get sign off for the final stage payment unless everything has been completed perfectly, so it'd be an idea to hold back some of your own savings to handle that. You'll also need your own funds for furnishing and decorating - they won't lend you money for curtains / blinds for example.