I am not currently working in Switzerland and am a UK national. Please can someone confirm that the Swiss tax Form 86 can only be submitted with all claims once per calendar year? Anyone know if a submission can be withdrawn and corrected?

You have 3 years after "having been eligible to claim" the income to claim back the Swiss WHT. Therefore, you can file in Form 86 once a year, no issue. Within that period, you can submit as many forms 86 as you want, for correction purpose or additional claim, whatsoever. As long as it is justified and correctly filed in...