Ruling out some of the options (some of which should be self-obvious):

(a) Using cascading delete from TableA -> TableB fails because parent Ids in TableB cannot be deleted whilst child rows exist.
IAC this would not deal with the deletion of child nodes in TableB so this was a non-starter anyway.

(b) Using 'INSTEAD OF DELETE' trigger in TableB

(1) With normal TableA-TableB FK Constraint I can't delete from TableA because it would orphan TableB items
(2) Can't apply a cascade delete because of the presence of the 'INSTEAD OF DELETE' trigger.
(3) Will work if there is no specified relationship between TableA-TableB and I delete from TableB using a 'AFTER DELETE' trigger in TableA

(c) Using 'AFTER DELETE' trigger on TableB - basically the same problem as (b)(1)

FWIW, a cascading delete from TableB -> TableC works OK.

Not having a specified FK Constraint between TableA-TableB (as in (b)(3)) would be a bit of a nightmare when it comes to creating the EF model. The only solution I can see is to define this FK relationship but use an 'INSTEAD OF DELETE' trigger in TableA which deletes associated TableB rows *before* it deletes its own rows, TableB still using the same 'INSTEAD OF DELETE' trigger.

A quick test seems to show that this will work - anyone see problems ?
It still means I'm going to have to tweak the EF model if I want it to perform cascading deletes on the entities - which is something I'm not looking forward to :-{