The AVA series handbook is only any good for general info, the AV is a totally different engine above the crankcase.
There are Series I and Series II engines, you need to sort out which one you have there.
Series I have the injector going into the head horizontally, Series II have the injector at a 45 Degree angle into the head.
The element has to come out with the holder part that has the gear around the body, if the rack teeth are damaged then it won't be easy. The actual element inner part should come away leaving the outer part in there.
Whitworth threads are the great-granddaddy of all 'proper' threads and should be treated with respect! :-))
Let me know which Series engine you have, and keep a lookout on ebay and abebooks.com for manuals, they are there most weeks.
Peter