I sent a special delivery letter Certified, HOWEVER the Royal Mail postal service returned it, "Addressee gone away". It looks as if theyhave shut the shop and run away.
if that's true, it looks bad for the Small Claims
Not normally a problem. The default position is that proof of postage can be taken as proof of receipt. This is the case, even if an item is returned to sender. The courts have been known to treat returned mail as "served".