A few years back Thomsons HO asked me about any parking concessions for the space above the Bull Plain branch as they were finding it difficult to let it. Ho ho. However might be worth asking if that space is still available. Also company have recently moved out of Tooke House next to Museum - worth an ask. Believe that there is some space in the offices next to the plumbing shop where Meldrums used to be.
Unfortunately much of office space in town has now been converted to residential - see Fore Street - apart from the huge premises as you have found such as Stag House, Rotunda. I believe one of the smaller tenants has moved out of Centurion House but with the uncertainty of what will happen re the Wrenbridge Regeneration (!) plans I have no idea what future may lie there Happy to ask the site manager if you are interested but it may only be for a short lease.
As a really left field suggestion you could ask the Town Council about the Seed Warehouse. It is a veritable tardis. My office was there and very pleasant it was. There are some tenants in situ I believe and there were some smaller office space but I have been out of that for some time and couldn't be sure how much of the building is now occupied and which bits.