There are at least three genuine Tompion sundials known. Two are at Hampton Court; one is a double horizontal dial which was damaged in the fire a few years ago, and the other is a large horizontal about 50 cm diameter - there is a 1950's replica of the latter in Kew Gardens, where the real dial stood for 150 years. The third (again horizontal, but smaller and simpler) which was presented by Tompion along with the famous month going equation clock to the Pump Room in Bath in about 1709 was lost for many years and discovered c. 1970-1980 by Meyrick Neilson, who gave it (I think) to the Pump Room; it is on a windowsill outside there now. Although that is much weathered it is very well engraved. The Hampton Court ones are fairly weathered but I believe the Kew replica is a very good copy and it is magnificent with an ornate gnomon and 1 minute division, equation of time table, etc. R.W. Symonds' "Thomas Tompion : his Life and Work" (1953?, reprinted Spring Books, 1969) illustrates two very fine portable sundials signed Tompion and (then at least) thought to be genuine, as well as the one then at Kew (showing the plinth and gnomon only as it is taken from the level of the dial) described as very fine work; there is a page or so about his making of sundials.
I don't think Tompion ever produced things that were not first class; his reputation in his lifetime and afterwards was pre-eminent because of his superb quality of work and design. Of course he ran a large workshop and also certainly signed clocks and other items which he did not make, as did most other makers - you can imagine him buying in a clock movement from Dan Quare to fulfil an urgent order or shortage of stock - but he obviously insisted on excellent quality. Although he is usually spoken of as the "Father of English Clockmaking" his contemporary reputation was perhaps even more as the supreme watchmaker rather than clockmaker and indeed "Tompion" became a word for a watch! He was also famously the subject of fake watches at the time, rather like some leading watch brands today. I'm afraid if this is a crude sundial it does sound like a fake, although who knows without seeing it? Pearson Page made all sorts of things - the water clocks are particularly notorious! Andrew James
