Leek, cheese & potato pasties

INGREDIENTS

• 2 small potatoes (about 300g/11oz), cut into 1cm cubes
• knob of butter
• 2 small leeks , tough outer leaves removed, split in half lengthways and finely sliced
• 3 thyme sprigs, leaves chopped
• 1 tbsp Dijon mustard
• 2 tbsp cream
• flour , for dusting
• 500g block shortcrust pastry
• 140g/ 5oz Welsh Caerphilly cheese, strong cheddar or vegetarian alternative, cut into small chunks
• 1 egg , beaten