JTQ, thank you for your explanation.
Having just returned home from undertaking the European Registration Scheme Gas course, under the auspices of CITO, and from the Caravan Industry Training Organisation Engineers Handbook, they give the information which I have previously posted, the hose is dated, but that is manufacture, and it does not deteriorate under correct storage.
Once LPG is passed through the hose, it does deteriorate, and it is from first gas use that the time limit is applied, with the recommendation of changing every 3 years being the best option, but at a max of 5 years.
Most service workshops change the hose or pigtail every year, just to be on the safe side, especially on 2004 onwards caravans which have the fixed bulkhead regulators, as they have 100psi gas pressure in the hose, unlike the cylinder mounted style which only have 28 or 37 mbar in the hose.
When I change a pigtail or hose, I affix a date label in the gas locker with change date and next due date for the owners information.
I do not know of any other workshop or service engineer who does this, but I do not know why they dont.