Correct washer/fitting for thermostatic shower head

[kingeric]

Hi there hope someone can help/advise on this matter, we have an orchard wye thermostatic shower shs7 purchased from victoria plum some time ago, my daughter recently took the square shower head down to clean it & has last the washer/fitting that sits in the hole at the back of the shower head to create a watertight tight seal between head & the swivel ball joint, I took the head to several local plumbers merchants who said I should contact the supplier so I spoke to victoria plum who can't supply the part I need & suggested I buy a replacement shower head from them but said they don't come with any washers which I find strange & then suggested I replaced the whole thing which I don't want to do.I tried a couple of washers approx 26mm & 28mm diameter as this isn't a standard fitting, please see pictures these might give a clearer picture of the problem I've got.