Bugs on thrunk of my Pittosporum

I have a Pittosporum tree that is covered in large clusters of black 'pin-head' sized bugs on the trunk. There are scars all over the trunk indicating that they are eating the bark. The tree has just started leaking a substance as well which tells me that it is not well.
Any advice would be helpful as I cant find anything on the Internet about possible treatments/explanations.
I've tried to upload photos but dont seem to be able to. Am happy to email them direct if you would like to look.
I live in the UK.
Thanks in advance...
David Lewis

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