It depend on whether you intend using the existing sky dish and lead to inside your dwelling or whether you intend to install a new aerial or indeed still have an original uhf aerial install. 

A Freely set top box may become available - but not yet.

There are a good Freesat boxes available from around £40 at Argos, Amazon, Currys, Richer Sounds etc

Or a used Sky box from eBay?

Freesat must have a satellite, dish bearing in mind that satellite tv is due to be discontinued and the boxes face extinction. The best future proof solution is probably to get a freeview box. It is a bonus if you have a tv aerial already fitted or if an indoor aerial would work( you would need to check this) again bearing in mind that a dodgy indoor solution is more than a snowy/grainy picture these day and gives wierd coloured block wipeouts and garbled sound. If your tv is still in good order and you have no plans to change it I would go with a freeview box that suits the budget. 

Terrestrial TV is under threat with plans to end it within 10 years and replace Freeview with Freely

Although you can use any generic set-top box to receive Freesat transmissions (as, indeed, you've been doing with your old Sky+ box), there's only one 'official' non-recordable Freesat box:
https://www.freesat.co.uk/get-freesat/set-top-boxes/4k-tv-box

It's widely available, including in Argos:
https://www.argos.co.uk/product/2075679

That box is 'smart', in that it can connect to your wifi to stream BBC iPlayer, ITVX, etc but it also has the Freesat programme guide for content beamed to your satellite dish.  (Freesat actually offers a subset of all of the Free-to-Air channels that are available from the Astra group of satellites but it's highly unlkely that you'd want to watch any of the missing ones anyway).

This looks like an excellent no frills box for the price.

Amazon.co.uk User Recommendationref=pd_ci_mcx_di_int_sccai_cn_d_sccl_1_5/257-5876321-6963645?pd_rd_w=m8N9F&content-id=amzn1.sym.6448d24d-f04d-4e11-90e5-10165d266aa2&pf_rd_p=6448d24d-f04d-4e11-90e5-1016

If your TV is already 'smart', go for the box that Togo has linked to.  If it's not, add smart capability to it by opting for the box that I've linked to.

A tenner's not much to lose, is it? Fingers crossed 

Thanks Dagman for the update, we are cancelling my MIL's sky next week and want to replace her boxes with reconditioned ones as she is 'used' to that system.

Feel a bit more confident now.

That's excellent, Dagman.  DDIL, why do you need to replace her boxes?  They will still work in the way Dagman uses his

She claims that they don't work any longer, although when we are there they seem to work fine.

If we replace them the 'earache' may go away.

Hopefully.  I think you said she has dementia, it's entirely possible it doesn't work because she is using the wrong buttons. 

Oh absolutely that is possibly the reason.

But we have written the instructions out several times with what to do, told her to turn it off at the plug and back on again - but apparently it does not work.

Mind you Anne up the road has posh sky and she thinks it would be a good idea to get what Anne has then Anne can help her with it!!

My oldster couldn't use the phone because he could never remember to press the green button even though it was writ large next to the phone.  He couldn't help it, he was unable to absorb information.  He had problems using the tv remote, too - he often didn't realise he was holding it upside down despite the bright red tape I stuck to the bottom.

Dagman, i do apologise for hijacking your thread in this way.

Barry, I think this goes back to are we being played/manipulated as when we are there the way she behaves/manages means there is no way she could live/manage on her own.  Yet according to nurses and neighbours she is a medical marvel!!

It' a huge problem for you, I know