Just under 2cm across - I presume not fully grown.
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If you take a look at the Vapourer at 8mm in ukleps you should see that its tufts haven't grown from their orange bases yet, as above. The 2 orange dots are on Vapourer as well and become more red as it develops.
...having looked at the ukleps pics: http://www.ukleps.org/Proms/2026s.jpg
The paired orange dots on the exact same part of the dorsal surface is quite a coincidence. Such a feature would more often than not be really distinctive and probably diagnostic. I wasn't at all aware of the feature on the immature Brown-tail larva until you pointed it out.
Lat/Lng: 51.6, 0.3
OS grid ref: TQ6097