Seb I had the same problem when I bought mine as the markings had gone. Nev or JW gave the name and phone number of someone in Plymouth a few posts back who has cheap props, give him a ring. The standard prop is 16.75 x 21". Suggest you have one of these and keep the present prop as a spare when its repaired. New from Lancing they are £300 (incl the VAT).
Nev did try a 23" but it was a bit tall. Better to a have a useable prop of 21" which gives 26 / 27 knots than over prop the boat loss the aceleration and strain the engine. 26 knots with good exceleration is much more fun to drive than 29 knots and desperately slow exceleration.
The trim tab on the back of the boat is for correcting the touque from a single prop but with hydraulic steering you shouldn't need it. If the boat is running down one side at speed either you have lots of weight in one side or a huge great big prop. Get rid of it. Once you have the correct size you can play with the trim tab but normally you need cable steering to feel the pull, might be a tad difficult to feel how much to adjust it with hydraulic steering.
Nev, brilliant way of getting a prop off,