Thank you, all, for your helpful replies.
A few minutes ago, I visited the DVLA page that I posted the link to above, where I clicked on the "Take off a number online" button. My understanding from what I've seen elsewhere on the DVLA site is that the existing number must first be removed from the Austin, otherwise it is likely to be lost. After completing the required fields to begin the process, the site came back with a message stating that my request could only be processed by post, using a V317 form. No indication of why this is so.
Rick F questioned whether age-related/reclaimed numbers on historic vehicles are all non-transferable. I think that's mostly the case, but maybe those allocated a long time ago were not subject to this restriction. My V5C definitely makes no reference to this.
On the basis that my Austin may need to be subject to an inspection and/or MoT, I can't see any means to transfer the ex-Morris 8 number to it immediately. I suppose I should therefore look into taking the number on a retention-only basis and transferring it to the Austin later, once I've got it back on the road and MoT tested.
I'm informed that the ex-Morris 8 number has now been removed from the modern car and held on a retention certificate with the owner. My main question now is whether I can acquire the number on the basis of keeping it under retention in my own name until I am properly ready to transfer it to the Austin. I can't find anything on the DVLA site that makes that particular point clear.
A few minutes ago, I visited the DVLA page that I posted the link to above, where I clicked on the "Take off a number online" button. My understanding from what I've seen elsewhere on the DVLA site is that the existing number must first be removed from the Austin, otherwise it is likely to be lost. After completing the required fields to begin the process, the site came back with a message stating that my request could only be processed by post, using a V317 form. No indication of why this is so.
Rick F questioned whether age-related/reclaimed numbers on historic vehicles are all non-transferable. I think that's mostly the case, but maybe those allocated a long time ago were not subject to this restriction. My V5C definitely makes no reference to this.
On the basis that my Austin may need to be subject to an inspection and/or MoT, I can't see any means to transfer the ex-Morris 8 number to it immediately. I suppose I should therefore look into taking the number on a retention-only basis and transferring it to the Austin later, once I've got it back on the road and MoT tested.
I'm informed that the ex-Morris 8 number has now been removed from the modern car and held on a retention certificate with the owner. My main question now is whether I can acquire the number on the basis of keeping it under retention in my own name until I am properly ready to transfer it to the Austin. I can't find anything on the DVLA site that makes that particular point clear.