90 degrees bend blocked drain

Did you try hot water and a plunger? What about going at it from the sewer AJ downstream?

Private septic tank? not sure if any acid based stuff is "healthy" to use there…

if you have/can get presure-washer, there are attachments for drain cleaning, something i got for myself few years ago
https://www.amazon.co.uk/dp/B0001P0BPI?ref=ppx_yo2ov_dt_b_fed_asin_title

Just match your power washer brand/model as connectors might differ

It's not exactly a 90 degree bends that's at the bottom as such - it's the design of the trap in the gully, it has a trap similar to a toilet bowl that will always be full of water which acts as a seal to stop gases, smell, fumes etc. from flowing back from the main sewer system. Goggle '' 4 inch wavin plain gully trap'' The photo will give you an idea of what your up against when trying to flex rods into it.

First remove as much of the water/grease as possible after agitating it thoroughly, make a ''scoop'' by cutting a plastic water bottle say 4 inches from bottom and tacking it on to a bit of timber. An old big ladle would be ideal for this.

Pour in 3 to 4 litres of hydrochloride acid (or similar) and leave for 24 hours, this should dislodge the remaining gunk. Needless to say you'll have to temporarily divert the sink/dishwasher water for this period

They should be able to give a price but it was 175 for me for a similar issue

Nope, not when there is sufficient water in the pipe.

You just need to be aware that this process is dangerous as you're making an acidic mix of unknown concentration, so use caution. I'd only advise doing this if there is a small amount of water bypassing the blockage, otherwise there is a chance that the acid just sits there and doesn't take effect. One of the issues with that can be that somebody else then puts their hand into the strong acid mix and receives burns.

If there is more than 2 or so liters of water in the pipe, add in around 150g max, so a quarter of a 500g bottle, and stir it gently with a wooden stick. Then either bin or rinse that stick very well.

Scoop out as much water as possible before adding acid solution.