Last winter at 126,000 miles the engine was a bit slow in starting from cold so I replaced the glow plugs, The old ones where a bit worn on one side so needed doing,

Not much of an improvement but with the weather changing for the better I didn't worry too much.

Showing intermittent codes of P0683-24 this summer, today I replaced the Glow Plug module ( not the relay since it's MY 2011), Easy to do but I am surprised it's so big. Thinking it's too early to see any change.

However in my research I can't find any way of testing the old one.

Plus I read that they should be changed every 60,000 miles.

Does anyone actually do this

I can't see why it would need changing other than if it's faulty. It's not a service item, it's an electrical component that should really last the life of the vehicle. 2009 Rimini Red SE TD4. Soon to be sold.
Hyundai Ioniq 5 Ultimate on order.
Gone. 2006 Tonga Green i6 HSE.