As far as I am aware safety and pressure doesn't allow user repair no gun spares are shown on any site.
so it's a replace job.
I take it it's the B&Q Performance power pro 1600pwa if so take the gun and one lance into B&Q with rought yr date of purchase as the bayonet fitting changed so you need the right type of gun. Look for Item number or means of id in gun. They may have them in stock or will order.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix general d.i.y.