big project for a DIYer! does the sliding door set have a timber frame surround like the rest of the door linings in your house? if so, measure the opening, halve it then go to B&Q and buy 2 of those. you might want to employ the services of a local carpenter...... if you cant find one, email me and i will put you in touch.....
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix windows and doors.