There is no evidence for an attempted German attack on the Suffolk Coast. German planning for an invasion of Britain focused on the south coast almost exclusively. The German plan, named Operation Sea Lion, was limited by the fact that the Kriegsmarine did not have much in the way of amphibious transport, and by the range of the Luftwaffe's aircraft. As such, they focused on the narrower part of the Channel. The Army called for an attack along a wide front, running from Ramsgate to Lyme Bay; ultimately, however, they had to concede to the Navy's objections and began planning for a narrower front in Kent and Sussex. An invasion of Suffolk was discarded because it would put the invasion outside the range of support of the Luftwaffe's aircraft, and because the Kriegsmarine would be unable to transport sufficient supplies to support the forces deployed. The site you mention claims that Sea Lion was a diversion, and that an un-named second operation was the real German plan. This is not supported by German sources.

There are several historical inaccuracies with the story presented by the site you mention, which cast doubt on its assertions. Firstly, it states that British radar systems detected the incoming German invasion force. This could not be done, as the British radar systems in operation in 1940 could not detect shipping. Focused on locating incoming aircraft at medium to high altitudes, it had trouble distinguishing between low-flying aircraft and the sea, let alone ships and the sea. Secondly, it does not mention a response from the Royal Navy. The RN was a key part of Britain's defence against invasion, and there was a constant coastal patrol around threatened areas of the coast, including the east coast. Harwich was a major base for the RN's destroyers, the main ships tasked with responding to any invasion, and the invasion route would have taken it across the heavily trafficked East Coast convoy route. It is implausible that such an invasion would not have provoked a response from the RN - no such response is present, either in the site's account, or the RN's logs and documents.

Finally, the reasons for keeping the 'invasion' secret are rather specious. The site presents essentially one reason: news of the attempted invasion would bolster the nerve of factions within Britain that wished for Britain to surrender, either in the Commons or in the Monarchy. There are two key problems with this. Firstly, the invasion failed; news of a British victory against a German invasion would only bolster British morale, rather than causing a panic. It would demonstrate not that Germany posed a threat to Britain, but that Britain could withstand anything Germany could throw at it. Secondly, many of the people the site identifies as likely collaborators with Germany were not. The site claims that Neville Chamberlain wished for Britain to surrender; however, during Cabinet meetings on possible terms with Germany, Chamberlain was one of the more vocal voices supporting fighting on. It claims that the Duke of Hamilton was planning to negotiate a British surrender with Rudolf Hess, Hitler's deputy who had flown to Britain in early May 1940 to meet with Hamilton; this would be a lot more plausible if Hamilton's response to Hess' arrival had not been to immediately notify the British authorities, leading to Hess' arrest and detention.