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Go to screwfix and buy a gallon of 'heavy duty degreaser', its about £7 and its the tits*
*for something that be used in the great outdoors on cars and such without killing all the ecology downstream of you, and not cost a huge amount/actually works. Its the concentrate that muc off/sh1t shifter is made from (loosely speaking). I wouldn't leave it neat on any finished surfaces for a long period of time, but for cleaning gunked up stuff its very good.
I use it in a plant pot spraying thing (you pressurize it, then squirt it out), low pressure spray over, leave to soak + agitate then high pressure shift it. Immediately rinse with a spray of water and does a good job without leaving residue.
What engine degreasers do you guys find work well? In the USA I came across gunk aerosol gel that was fantastic but when I ordered what was meant to be the same thing here it was fucking useless and I ended up using gt85 with better results.