Aircraft requirements to fly in Airways
 

I am struggling to work out the minimum hardware required to fly a private SEP in Class A airspace.
I have looked at CAP393 and believe the following is required:,
ILS, DME and an ADF.
display of outside air temperature,
transponder has Mode S elementary surveillance,
antenna diversity (whatever that is)

The requirements are specified inCAP393 tables:
Section 1, Schedule 4, Page 2
and
Section 1, Schedule 5, Page 1 (Radio comms and nav)

Could anyone advise if this interpretation is correct and if exemptions are available for Mode S for example?

Many thanks

If you want to fly airways you won't need an ADF in practical terms but you will need an IFR certified GPS to meet RNAV requirements and ILS DME for approach and landing as well as the outside air temp, twin altimeters and Mode S transponder. You will also need an 8.33 capable radio.

For questions like this, I think you'd better look in the AIP of the relevant country. The GEN 1.5 section to be more specific.

For the UK, the information can be found here: http://www.ead.eurocontrol.int/eadba...2013-12-12.pdf -- Para 5 (page 10 onwards, and particularly the table on page 11) seems the most relevant to your question. Although I have to admit it's a rather dense document.