So, I have waited for the replacement cost of my P3P to fall considerably prior to thinking that addiing a GPS tracker would be a good idea.  Or, to be fair, I didn't realise that it could be done.
Could anyone with relevant experience please offer some advice?

This might help you:
https://phantompilots.com/threads/best-budget-gps-tracker.107933/

The Whatsapp Tracking can also find misplaced devices and keep track of reports and events. Global positioning systems are used by third-party tracking apps. These GPS satellites allow the apps to pinpoint the exact location of any device connected to their network. If you misplace your phone, you can track it down in seconds using any internet-connected device.

WhatsApp tracking only works phone to phone so for a drone you'd have to strap a phone to it, turn it on, open Whatsall and then connect to it using your phone Only then will the drone one be able to tell you where it is.
If it goes down you not only lose your drone but also a phone!!

Not a good idea me thinks.

Ideally, a drone will crash and the tracker will send its location after crashing.  Or a drone will crash and the tracker will have sent it's location a second before the crash which will be logged on your mobile device.  Since Flytrex turned off their servers in Aug 2017, I've been using Trackimo.  It's not nearly as good as Flytrex as it only updates once a minute unless you manually request an update.  And it doesn't always connect with the cell towers which means you will often have several minutes with no connection (Flytrex updated twice per second and didn't have cell tower connection problems).  Nonetheless, it is possible to track the Trackimo location live in real time which can be handy provided you manually request the location, although you can only do it 3-4 times per minute since it takes 15-20 secs per request.  I've crashed twice with the Trackimo, and neither time did it help me locate the drone.  The first time, the Trackimo turned itself off on impact with the ground, although the device wasn't damaged.  Turning itself off on impact is not helpful.  The second time, the drone crashed near a barrier wall and so the Trackimo couldn't get satellites with so much of the sky blocked and so couldn't give me a helpful location even though I had the cell tower connection.  Frustrating.  Nonetheless, I use it anyway because I don't know of one that's better and I figure it's better than nothing.  Theoretically it would work but it has failed me two out of two times, although it did give me its location prior to the crash which is somewhat helpful.  I keep hoping somebody will develop a good tracker because Flytrex proved the tech is there - somebody just needs to figure it out and do it.  Annual fee for the Trackimo is around $60 a year.  Good luck.

Seems like a lot of money for not a very reliable product. The GPS record from the drone is pretty good so I think I'll stick with that

The GPS record from the drone to the mobile device is good if you have a good downlink connection.  If the drone dropped, it should provide a decent idea of where it dropped.  The GPS record as recorded in the drone itself is excellent, of course, but you can't read it until after you recover your drone.  The advantage of a tracker is that you can potentially read the location of the drone in real time even with a broken RC connection, such as during a distant out-of-range autonomous mission or a close autonomous mission behind an obstacle such as a large building or large hill or thick woods or a mountain, or even during a fly away (but I've never experienced a fly away).  

"Scotch Whiskey - I always take it at night as a preventative of toothache.  I've never had the toothache, and what if more, I never intend to have it."  -Mark Twain

cheddar-man Posted at 10-17 07:01
Seems like a lot of money for not a very reliable product. The GPS record from the drone is pretty good so I think I'll stick with that

The GPS position from the drone is only good as long as there is or was a radio link between the drone and your RC. If you have a flyaway, and lose radio connection, it is no use at all. The Marco Polo is one of the most highly recommended devices as it is completely independent of the aircraft.

The only Marco Polo I can find here (UK) is for tracking pets, cats, dogs etc???

Yes -  would try a Marco Polo if I didn't already have the Trackimo.   https://eurekaproducts.com/rc-model-tracking-and-recovery/

Nice MTD but this old pensioner can't afford that sort of dosh on a part time hobby! Any way, it's covered on my household insurance so I'm told!