I've previously seen Amanda installed in such a way that you could run
something like

amrestore -p /dev/nst0 host /filesystem | restore ivf -

Which gives a basic shell on the tape so you can restore individual
items.  I understand that this was possible because comp-root backup had
been used on an entire filesystem, as I've tried on a sub-directory (as
is needed here) and it returns an error which google lead me to a post
on zmanda explaining comp-root can only be used on filesystems.

So, with the current set up its using comp-root-tar:

define dumptype comp-root-tar {
    root-tar
    comment "Root partitions with compression"
    compress client fast
    index
    exclude list "/var/lib/amanda/exclusions.txt"
}

Is it possible to restore from this backup interactively or is it only
possible to untar the whole dump from this particular DLE, then take
what I want then discard the rest?

If its not possible to do it interactively can anyone suggest a way of
backing up a subdirectory on a filesystem then restore particular files
from that backup?

Many thanks,

john