To want to go in my garden without my neighbours being intrusive

Our neighbours are like this. They're an absolutely lovely retired couple. I really like them and have invited them round for a cup of tea many times. However, sometimes I just want to nip into the garden and be there in peace but they always rush out to chat. Once we chatted for two hours!! And I just couldn't find an opportunity to get away.
I don't know the answer to this problem.

MN ( me anyway ) likes pictures / diagrams in these situations.
It might help with suggestions OP

as an aside we’ve finally decided to just move because of our neighbours. 30/40syrs ( as you mentioned age ) and extremely aggressive. We park 1metre from the front door, check our ring doorbell to make sure they aren’t wandering around and then dive into the car. I haven’t walked down the drive in 3years so the weeds are now really high, it’s so embarrassing. We get our sons to mow out the front. It’s horrible. Life’s too short.

I second the headphones. Just get cheap ones for £10, shove them in and go out. When they speak to you ostentatiously remove one headphone from your ear, and say, “Hi Bob. Couldn’t hear you. I’m listening to a course/lecture/work.” Then turn your back and ignore them.

Strategically placed shade sails?

This is beyond annoying, now granted I don't have it anymore (apartment) but it's so intrusive & actually forces you to become either a "rude person" or a silent fuming angry one & it ruins the enjoyment of being outside enjoying your garden

Well - yes. That is what you need to communicate to them.

I'd start by interrogating them for a change.

'Bob, every time I'm in my garden I can guarantee you'll come out and start nipping my ear. Why is that? Are you actually watching and waiting? Because I can't see any other way it can happen. That's really fucking creepy, Bob. Really fucking creepy.'

I might move on to -

'I should log your behaviour. Date and time that I or DH come into the garden, how long it takes you or Roberta to come out and start nipping away. Really fucking creepy, Bob. Really fucking creepy.'

Fence
Trellis with thick growing plants
Hedge

You can buy pre-grown hedges quite high these days.
Id still go for a fence and planting myself though

short term hassle will lead to long term enjoyment of your garden

I’d tell him you’re finding him intrusive. Just say ‘Don’t mind chatting out front but in the back, we want to get on/do jobs/have peace’. Don’t apologise, you aren’t in the wrong for wanting some bloody peace!

Just carry on with the task that you're wanting to do, so hello (don't be rude to them) and work away - engage in conversation and they will never get the message.

Put trellis on the top of the fence. Plant tall shrubs, preferably prickly ones

Look, if someone doesn't get a hint, you need to be more direct.

"I can't chat with you Bob, I need to do this."

Turn your back and get on with whatever he's doing.

If he continues talking to your back, ignore and keep ignoring.

He will stop eventually.

People who behave like toddlers need to be treated like toddlers.

Excellent!

You will have to be blunt which may not be the ideal.

Oh god my neighbour is/was the same! It doesn't matter what we were doing, he'd pop up and simply have to offer a bigger tool / a better decision on what needs doing/ a bigger, better everything... and he'd love to chat for hours! He always knew what was going on in the street (cos he'd go over and ask/stand there while they're doing whatever - and giving his advice)

He was a 'Cheeky Chappie, everyone loves me, right' character, but he went too far last year and really pissed me off, to the extent I told him I'd go to the Police if he spoke to me again. (Sexual harassment - these people always seem to think I welcome their opinion on my clothing, hair, size, looks, sense of humour...)

That may be the nuclear option, OP. Worked for me so far.

Oh I would hate this - had some neighbours in our last house who always wanted to chat - very tricky, really put me off going in the garden. We moved house (not because of that) and it is very nice now to have a more private garden

My sister had this with her neighbour, she put up a huge fence.

I could understand your problem if they are sweet people and you don't want to offend, but given they aren't very nice. I'd ignore or engage at such a low level they wouldn't bother. Failing that I'd be really blunt and say I haven't got time to talk.

I'd do a Larry David and be blunt/honest. I'd tell them that m extremely private and feeling a bit suffocated because of this obligation to have to chat more frequently than is comfortable and that in the back garden you need to be left alone. No room for misunderstanding. I've done similar a fair few times in my life....it has worked. Best of luck op

If you can't have fences, you need to look into some kind of plant barrier.

Also, if they're always complaining then your relationship can't be that good anyway? In which case, it doesn't matter if you offend them by telling them you don't have time to speak to them.

"I'm so glad you're here.
Can you come and have a look at the dog?
I think he he's got rabies."

Why can’t you do big headphones but have nothing playing- they don’t need to know there is nothing playing