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Should have been clearer, I'm re-fitting the kitchen with new cabinets and I'm just re-thinking the waste layout before I refit, it's not actually blocked (although has been before and it is totally shitty, with very little drop).
I'm going down to a 600mm cabinet below the sink in order to cram a dishwasher in so under-cupboard space will be at a premium and there's a nice big void behind where it may as well sit.
I assume it's a bad idea to conceal a u-bend completely behind a cabinet? If I trim the waste back a bit I can fit everything behind the cabinet. Obvious solution seems to be to make the panel at the back removable incase anything fucks up in future.