Paul,
As Colin & Piper5 have pointed out, the Vulture call signs are all SORC radios - so they are your contact. Graeme Barry is the man (email is graeme (at) graemebarrie.com)
If you want to have the hand-held as a second radio to the car-unit, then it will get its own ANI number, and become Vulture 44 Alpha.
Alternatively, you could register the radio through SLROC, get both a call-sign and an ANI number through James (steve.james2 (at) uk.bp.com)... and the radio would be LowBox
some-number.
Personally, I'd not recommend the second route: 1) You have to remember which radio you are using, and remember to switch from Vulture 44 to Lowbox
some-number when you pick up the hand-held; and 2) It will confuse the heck out of the event officials, until they remember that you have two different call signs.
Of course, there is a third alternative: switch BOTH radios to SLROC, and they can become LowBox
some-number and LowBox
some-number Alpha