I can't remeber whether it was the 15th or 16th edition regs, but the number of recommended fuses for BS1363 plugs was reduced. Out of 3A, 5A, 10A and 13A, the 5A and 10A fuses are "non-standard" and 3A and 13A are the "recommended" ones. This is disputed by some elecricians and it makes me uncomfortable.
The fuse value chosen is decided by the manufacturer of the equipment, determined by the feed cable size and the load characteristics of the equipment. For low current devices (below 700W a 3A fuse is usually used along with a 0.5 or 0.75 sqmm cable (a manufacturer may recommend a 5A fuse to prevent nusiance fuses blows based on surge or start-up currents). Above 700W a 13A fuses is usually recommended.
If your PAT tester was testing according to the 3rd edition of the IEE Code of Practice, he's actually deviated on two points, but I know why and I agree with him to a degree; I'll come back to this in a minute.
The IEE code of practice states for formal visual inspection: "The fuse recommended by the manufacturer should be fitted." and for combined inspection and testing: "inspect the plug as for a formal visual inspection".
In appendix VII, Checks to be made on a plug, a cable and an extension lead: "Check the rating of the fuse. The manufacturers recommendation should be followed." and "Most appliances up to about 700W should have a 3A fuse fitted (red). Appliances over about 700W should have a 13A fuse (brown) fitted."
I presume that Jo-Jo have chosen the 13A fuse to deal with heavy startup/inrush currents that might be present on some appliances used with the extension lead (remember not a fixed appliance-type here). A 13A fuse should protect a 1sqmm lead against fire (if unwound) -it may get hot, but the fuse will blow before the cable melts. A 10A fuse would protect it better, but the 13A fuse is within regs.
I bet the reel is clearly marked with "Max rating 10A" and "Do not use above nn Amps unless fully unwound", thus placing clear duty of care on the user to keep within safe limits
I would pass this item, but put a note in the results that a 10A fuse would be safer unless nusiance blowing is a problem. Being a Jo-Jo, I bet it also has an integral overcurrent breaker that will trip if more than 10A constant current is drawn (different time characteristics to the fuse to allow for transient currents). I do worry about 13A fuses in 0.75 sqmm (I just fit 5A and record it) or 1.0 sqmm IEC (kettle) leads though, as you never know which is as supplied and which is just user replaced.
Sorry if this appears confusing, but I hope there is enough info to take your PAT tester to task with this item. If he's failed it on a another aspect, that would be a different matter though.