Bike trailer_wheel fixing part has been lost.

[kwaszak0]

Me and my family just started with 'bike trailer trips madness' and unfortunately we lost one very important part from our trailer.
Picture of it is attached.
I'm just wondering if you could help us to find online shop which can supply this part. We live in Southampton, United Kingdom.
Any help is greatly appreciated as we are planning longer trip to Isle of White soon.

Thanks in advance.
Przemek with family !