Just shows that It is not always the "usual suspects" that have problems!
£13K sounds a little on the high side for a non-factory O/H but not if the engine core items could not be used (I take it that it is a Lycoming o-360).
There is only one non-factory engine overhaul company that I would use in the USA as most of them will walk away from a problem child safe in the knowlage that you are the other side of the Atlantic and unlikely to come knocking on the door.
With the £/$ rate as it is at the moment a factory overhaul looks like a very good deal