As above, get some sort of degreaser / cleaner and a dressing product.
I use Meguiars APC (All Purpose Cleaner) and Chemical Guys Bare Bones for dressing.
Here's a shot of mine after 10 minutes work (granted its a 59 plate car so was THAT dirty in the first place).
Before:
After:
You can use a jetwash on a wide angle spray pattern but dont direct the spray straight at the engine, sort of aim over it... Give the whole engine compartment a good soaking with a degreaser or cleaner the use detail brushes to aggitate the product in... leave for a few minutes and then with the engine running rinse (i dont cover anything up but thats a risk i am happy taking). shut the bonnet and let the engine idle for 10 minutes then dry off any remaining water. Lastly liberally cover the entire compartment with the dressing product and then shut the bonnet and leave it. Come back a couple of hours later and buff off any remaining product with a MF cloth.