You generally get a bit of seepage over a couple of years, especially if you don't use it over winter every now & then, King Auto Electrics in Bayswater have topped up heaps of mine over the years & prices are pretty good too, it sounds like it's low on gas but luckily not empty or the compressor won't cut in at all, so no huge leaks.
A lot of aircon places will insist on evacuating the complete system & replacing bit's & pieces, costs a lot more than a top up & usually not necessary, they just make more money out of you.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.