Flash: Nissin D1622 - cheap (100 quid), well made and does the job.
Lenses: if you don't want to hire or buy then you can shoot the whole thing with your 50mm 1.8. But remember that your feet are the zoom.
Wide angle lens will be useful for groups - but watch out for distortion.
Manage expectations. They aren't paying 2 thousand pounds to have the wedding photographed so getting it done for free comes at a price. Don't try and do everything. Promise less, deliver more.
Make sure you shoot in RAW so you have maximum flexibility in post should you screw anything up. It also enables you to correct any White Balance issues.
Take back-up batteries, flash batteries and memory cards.
Check your camera settings on a regular basis as you move in and out of locations.
Good luck