I'm also not familiar with UK law, but here are my 2 cents:
first of all, you mention it is a design patent (something that does not exist as such in Europe/UK, it is either a design registration or a patent) and you refer to the website of the OHIM.
Thus I suspect we are not talking about patents here, but about a design registration. There are different types of European design protections and an interesting article on designs in Europe can be found here:
http://www.mondaq.com/article.asp?articleid=61252.
In general a valid European design right (I suppose this is what we are talking about here) can only be obtained if the design is new and has an "individual character." Novelty is defined in slightly different terms from many national statutes in that designs which differ in only immaterial details from a prior disclosure will not be regarded as being new. Individual character means that the overall impression of the design is such that "it produces on the informed user" a different overall impression from that of any prior design that has been made available to the public. OHIM officials have indicated that what is meant by an "informed user" is someone intermediate between an ordinary user and an expert. For some types of products it has been suggested that such a person might be a retailer of the product in question. In making a determination of overall impression "the degree of freedom of the designer in developing the design shall be taken into consideration.
From the above, and from the knowledge that your product was already on the market when your competitor filed for design protection, it can be concluded that either:
a) the registered design is not new over your design, and thus is invalid
b) the registered design is new over your design, but then your design will not be covered by the design right.
Thus, if it is true that your design was around before the date of filing of the competitior's design registration, then there will be no problem.
On the other hand, I know that in the UK it is very unwise to send threatening letters in IP cases, since such an act could cause liability. Maybe this gives an opening for you to file a counterclaim.
Nevertheless, I suppose you better spend a few quid in talking to an IP lawyer.
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