R D S wrote:

Ideally I need to connect a sink waste to a soil pipe, it's 3 inch.
Does a 3 inch strap on boss exist? Because I can't find one.

definitely 3" or could it be 82mm ?

And if I can't find a suitable boss....
Would it be a terrible idea to drill a suitable sized hold in the soil
pipe and just ram the boss adapter in?

probably.

By adapter i'm talking about this,
https://www.screwfix.com/p/floplast-...m-5-pack/89174

Also do we care where we connect it?
There's a few inch of pipe indoors under the lav, gut feeling is to
connect it as far from the actual pot as possible, are there rules?

if the flow from pipes is "opposed" into the stack, there's a minimum
offset, if not opposed, I don't think there's a limit

Part H will tell you

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf