Please see the notice below from the National Bee Unit.
Some of you may not have gotten round to treating your colonies with oxalic acid as the weather was so mild in winter. Treatments that were applied in winter may have had lower than normal efficacy due to the presence of brood and therefore beekeepers may want to consider treating colonies again, especially where bees are showing signs of deformed wings. Thymol based products and formic acid pads may be ineffective at the present time as daytime temperatures respectively of 12-15 °C or above are recommended. Neither should MAQS strips be used on smaller colonies.
Therefore contact strips such as Apistan or Bayvarol may be beneficial, these offer a rapid knock down in severely infested colonies. However, resistance to these products has been reported in some areas and therefore colonies will need to be monitored after the treatment and an alternative treatment applied if necessary later in the season.
Alternatively, Apivar & Biowar (Amitraz) are available under the EU Cascade system by using a special import certificate. For more information about this, contact your local vet.