I've been told to expect between a 1:00 - 1:20 flight at standard aircraft hire rates and a £50 fee for the examiner. It really depends on what the examiner wishes to test. Yes he/she can include navigation but as you'll notice from the table in Appendix 3, this is not mandatory.
You might be better off asking the CAA if any JAA examiner can conduct the test according to Appendix 1 & 3, sign your SEP(L) rating and complete the paperwork that goes off to the CAA. Or if it must be CAA examiner.
Perhaps what the IAA tell you is correct, that you do a skills test to re-new (not re-validate) your expired (by < 5yrs) rating. This is the same as the UK. Sure the examiner can make you fly an entire test if he/she wants to but it's at his/her discression. I would expect him/her to adjust the extent of the testing to reflect your recent flying experience, but always incude the mandatory items (of course).