These are the washers
M6 (6mm x 30mm) Flat Repair Washer (Large OD) - Stainless Steel (A2) (Pack of 20) : Amazon.co.uk: DIY & Tools
The wheel lasts longer when used upside down I find, because when used the right way up, after time it starts to lean on the stand and rub against the base - doesn't happen when it's used upside down. It's also very secure.