I notice Ren you have no replies to your question. Well, looking out of my window its snowing hard, its warm inside , I'm retired and although I have plenty to do outside in the garage I can't face it today, so I will share my thoughts with you on this difficult subject.
Firstly, glad the car is in the garage that helps no end. Valuation is not so easy it all depends IMO on the condition of the body and structural items underneath. Mechanical items are one thing the sheetmetal work the another. You know probably as well as me the time factor to turn a car that really needs bodywork into something very presentable can be a very expensive matter. If I were to access a cars value the bodywork condition would be my highest priority.
Is the car matching numbers with known history? The 356 will run well with all sorts of modification to running gear but the running gear has to be seen to be mounted to good sound metalwork. It all comes down to that. Is the interior original in good condition for its age or has been well restored?
Your valuation of £20K I guess represents a car that runs well has an MOT and looks nice from a few feet away in other words a nice driver. If you could do the bodywork yourself and spray it and if the history is there the value might increase considerably depending on how its done. If you have the work done professionally and it really needs a full stripdown etc, who knows to cover the cost you have expended the value then should be around £35 -£40K?. But, do you think your car would actually make that amount on sale?
How many photo's have the insurance company asked for of your car? They must make their considerations on these and the market value for the model and type.
The value for agreed insurance is the amount needed to replace the car with another I guess in similar condition in the event of total loss. If you really think your car is worth much mor than £20K maybe you should get a known independant 356 expert to put pen to paper for you ?
Its almost impossible to really make a judgment unless the car is fully inspected, photo's lie in many cases as the insurance people probably are aware. ( I can tell you, my car photographs very well, and even with high resolution they still look good until you put your face 6 inches away and start to look then its different.)
So Ren, thats why probably you have not had a lot of answers.
Hope you sort it out but if not, keep it at £20K take it easy and enjoy the drive, thats what I do. By the way your car looks very nice in the photo.
Roy