Non interventional post-authorisation safety study (PASS) to define the incidence of and potential risk factors for procedure-related neurological adverse events (AE) in patients undergoing ultrasound guided foam sclerotherapy (UGFS) for the treatment of lower limb varicose veins (VV) using Fibrovein 3% and 1% (sodium tetradecyl sulphate, STS, injection) incorporating a drug utilisation study (Fibrovein PASS)