You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
I just decided I'd do a quick aircan service and whipped off the rear shock....
Somehow I lost the black "mushroom" bit... (in the grass) ... 2 hours later and ripping out grass still can't find it.
I was looking forwards to riding the Whyte this weekend... and I can order a new mount for £7 from Whyte but was really planning this long weekend a bit of T-130 stuff...
I could stick in some steel washers to space it out... unless anyone has an idea how to do this by tomorrow?
Tried several local Whyte dealers... 🙁

As you say, I think if you are adamant on getting out on it tomorrow, then you'll need shim it using steel washers. I certainly wouldn't ride it without shimming it.
I'm wondering whether the counter sinking of the dome washer has a purpose? That might be your catch, as you can't replicate it with basic steel washers. I'd just pop it all together with the steel washers and then review how functional it is.
Some washers about the same diameter to space out the load on the frame should be fine. The counter sinking can't be for anything other than aesthetics and to stop clothes/skin snagging on the bolt head. I wouldn't worry.
Ring Banjo Cycles in Newbury.
Cheers all .. Now tried Banjo (thanks Weeksy) AW Cycles @ Reading, East St Cycles, Spokes @ Bagshot and Cycleworks @ Guildford (since they are close).... but someone on FB Swinley Group came up with the idea of using an empty vacuum which I thought was pure genius!
haha. Empty vacuum idea is great indeed!
Good luck and get your bike going again.
(Make sure you get the "play" out of the mount. If the mount "rattles" - don't bike it...- in this case...better wait for the correct mounting hardware...)
The Vacuum worked ... shock now serviced and refitted.
Thanks everyone for ideas but in the end that vacuum idea was great ... now I’ve got to clean the grass and stones out of the vacuum but hey I got a working bike.