I suggest you answer the letter, it makes no difference that they'll know where you work - they know already because they sent the letter. Ignoring the letter will only prompt them to write again to your work and you can't run the risk of your employer finding out what your business is.(posted 8274 days ago)If I were you I wouldn't mention harrassment at the moment. Keep your letter simple and polite. If DLA carry on writing to you at work, then you can play the harrassment card.
As for the delay in writing to you, who knows? My guess is that like most solicitors chasing shortfalls, they think that you'll give in immediately, but you didn't and they had to go back for instructions from their clients. So your lender decided to leave it for a while, and your file has gone back to DLA and they're treating you like a new victim rather than realising that you've been down this road before. As already advised, do ask for a reply to your previous letter.