Tamron 90mm f2.8 (£350), Canon 100mm f2.8 (£420) or Sigma 105mm f2.8 (£370) are the front runners, but you could also use extension tubes (Kenko 12mm, 20mm & 36mm £100) on something like a 70-200mm or a prime lens like your 50mm which is an alternative to a dedicated Macro lens.