clematis - something eating the leaves.

Could be earwigs. They like clematis. I spray with soapy water which seems to help a little.
Is the plant growing in a pot?

Yes it's in a pot.
I'll try the soapy water.

Quite likely leaf - cutter bees, they cut semi circular pieces of leaf, then bury them in loose soil where they lay an egg. They feed the larvae with pollen, which eventually pupate and emerge as adults. They seem to really like rose leaves for some reason! They are also a nuisance in the greenhouse, because they like to burrow into pots, I have quite a few at the moment.

That sounds like it, Nan. I've seen a few unusual-looking bees around, in and out the open windows etc.
Does that mean that the contents of the pot need to be changed? If so, it's a big job.

No, once the adults have emerged, the leaves ( which are rolled into tubes like little cigars) just rot. I have many hundreds of cacti, and many are targeted every year, no way would I repot them all, although I do find it very annoying!
The bees are very determined in their activities, I've sometimes just moved one of their target pots to a different area, and they get quite frustrated in their efforts to find it!

We had cutter bees which made large, well-defined holes in a pot-grown camellia. Then the plant got a black mould all over it and we thought that was the end of the camellia.

However, I washed every leave in soapy water then planted it in the ground and it has been fine every since.

DH reckons it's probably vine weevils. Have a look with a torch after dark. If you see the little rotters the only thing to do is pick them off and get rid of them.

Someone mentioned vine weevils to me when it happened to the camellia, but the 'cut-outs' on the leaves were definitely made by leaf cutter bees. I'll try to find a picture then you can compare the damage Fennel. Vine weevils will destroy the roots - they killed off my heucheras which were in pots.
This is a rose leaf:

You talk about Holes in leaves! Have I got holes in leaves!

This plant was a freebie from Parkers. AKA an odd plant they can't get rid of. It really needs chucking out. I don't know why I keep it. It looks like caterpillar damage. It is supposed to be a Hydrangea Annabelle and I have had it for two years. Be afraid!!

Can anyone tell me if in your experience Clematis
(Montana) have a limited life span? I had a very vigorous plant climbing etc and seeming to do well and then one year it simply did not grow and had died.

I've a common or garden pink Montana which I must have had for over 20 years. It is completely rampant and survives everything, even being cut back hard!

I'd suggest that yours might have succumbed to a disease or a pest, though what disease or pest I've no idea...

I've had the odd supposedly indestructible plant die on me quite unexpectedly (one was a lilac, for heaven's sake! Do lilacs ever die...). Perhaps it just happens from time to time

I've lost several plants in this garden which seem to flourish like weeds in other gardens!

That hydrangea looks a bit Nelliemoser. However, I never give up until a plant has really gone. I'd spray it, chop it back a bit, give it a mulch of shrub compost and hope for the best.

I hope it's not one of those new super-caterpillars eating it (is it the diamondback moth caterpillar?)

Jalina1108 I have not seen any such nasties before . Nothing else in my garden has been attacked to that degree. I have a red hydrangea that has dark red leaves and petals and that has not been shredded to that degree. I wonder if the red plant tastes bad to the pests, what ever they are. I might just spray it.
Would you like me to send it to you so you can see if it does better in your garden. (Seriously)

I once found this caterpillar chomping on a potato plant:
www.wildlifeinsight.com/20583/deaths-head-hawkmoth-caterpillars-in-the-uk/

Perhaps not, thank you very much, Nelliemoser - whatever comes with the plant might start eating my hydrangeas!

When my camellia was eaten then smothered in black mould-like stuff I really gave it a good wash in dilute washing up liquid - kill or cure!

I'd never heard of leaf cutter bees.I now suspect them of eating a lovely large leaf (name escapes me) plant in my garden. Id blamed slugs - apologies slugs.

Never apologise to a slug!
It will just think that it has permission to eat its way through your garden.