Domestic sewers cause few problems , commercial kitchens dumping lots of fat are a main problem especially in central London.
I personally wipe the pan first with kitchen roll
the grill has to come up.
if the pipes are in the way, you could cut them and rejoin afterwards. I expect they are plastic which is easy to cut with a fine-toothed or hacksaw.
It's too early to say that you have to pay hundreds to a squirting company.
An older plumber or handyman who doesn't find stinking filth can do it.
The Drain will have a right angle bend as a trap to stop smells etc, you could try pushing a flexible hose pipe into the gully and try flushing it through,
where is the nearest Manhole Inspection Cover, do it the Old Fashioned way get a set of rods and rod out the drain from the inspection pit whilst running water into the gully
Borrow a wet and dry vacuum cleaner and suck the mess out, then you will be able to see the exit from the drain. Better still buy one as the paint erson you borrow from may not want it back no matter how hard you try you clean it.
but the rods are safe and don't damage anything
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