Tanks needs to be tested by a 'competent person' to meet obligations under the Pressure Systems Safety Regulations. Often they are inspected on each fill and tested at 10 and 20 years but the duty is that they are checked in accordance with the owner's (usually the LPG supplier) 'Written Scheme'. Such schemes have to meet legal duties but can vary widely. If you are concerned talk to your LPG supplier.
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The trade association for the LPG and bioLPG industry in the UK