Enlarging a Window / Removal of External Door

Hi!

There's a window in our kitchen which I would like to enlarge. Directly to the left of the window is a back door into the garden which we do not use as we have another back door elsewhere.

I would like to have this useless back door removed and the window widened to take up the space. The missing bits of wall would need to be put in also, obviously.

Anyone know who I should talk to for a job like this and what kind of € it's likely to cost?

Thanks!