Anyone got any idea how long it would take to obtain permission for an underbuild in Hondon - or am I just living in Cloud Cuckoo land based on the Town Halls current reputation?!

It's difficult to say but based on our experience, working in the construction industry, each Town Hall interprets the laws in it's own way. However, again based on our experience, depending on what you want to do with your underbuild, you might be well advised to instruct a Spanish Architect who will usually know someone in the Town Hall and get things moving more quickly for you.
Hope this helps.