Thank you to everyone who've responded to my enquiry :-)
It is good to know that this is a common problem & not unique to me.
Re feedback from contributors 'jdow' & 'Carlos'.
I note with SDR# that under the AUDIO heading that the Sample Rate is always greyed out at 48000 samples/sec.
Can I somehow change this in a setting elsewhere to see if I can get rid of this spur on 98.304MHz?
My RTL-SDR device as used with SDR# allows me to change the sample rate, but seemingly not the AIRSPY HF+.
Has anyone had any luck with excising this spur yet?
Feedback would much appreciated.