Hi EGOL,
Comparing our traffic for that term to the previous year, we saw a 5%-ish drop every year around March (seasonally, people don't want raidators when it's warm so thats fair). However, this year on or around March 21st we saw a massive hit, which continued into the beginning of April. This affected many of our key landing pages such as Boilers, Radiators, Bathrooms, Shower Enclosures etc. In this time we did not make a major change to the site, but continued to use the SEO practices that had always worked for us.
This makes me think it's down to an algorithmic change from Google but according to many SEO tools and people in the business, the things we are doing are correct, and our site is 'stronger' than our competition.
It is just very frustrating when our business has relied upon our strong positions in Google - and we haven't rested on our laurels - but it's as if we have been penalised.
Sorry for the moan and thanks for your advice so far!