£5 admin should not apply if you decide not to travel due to cancellations, how the GWR▸ website implements that (or if it does at all) I don't know.
Indeed. Should be a full refund. Once you get the refund, check the amoint and if it's £5 shy email GWR asking for the balance.
I did note that at one stage I was asked to give a reason for the refund request, assumed that the £5 admin fee reference was a "catch all" statement for other unspecified reasons.