De’Longhi EcoDecalk Descaler for Coffee Machines DLSC500, 500ml £7.81 at Amazon

I just use a table spoon of citric acid which does the job and much cheaper

I find the HG coffee machine descaler does the same job for my Delonghi Dedica and it seems to hover around the £4 mark.

I've been using Big K Descaler for my Delonghi coffee machine and had no issues. Bought two 400ml bottles from Robert Dyas for £7. Each bottle should cover 5 cycles. (edited)

Any decent descaler can be reused several times. I collect the discharged warm fluid from the machine in the kettle so doing two at one sitting. I put a twice folded sheet of loo roll into the narrow end of a funnel and filter it back into the bottle.
When it takes more than 5 mins to clean your kettle then it is time for another purpose, so take down your shower curtain and put it in a bucket with the old mixture and let it soak. Once that is clean then wait until bedtime and pour the mix into the loo pan to sit there overnight.

I am not denying this is a good price for that brand, but that basically what you're paying for, the brand. I
I switched to Ecozone Coffee Machine Cleaner and Descaler
It does exactly the same thing and is only £4 (£3.8 with subscribe&save)

£7.03 on subscribe & save.

I knew a woman in Louisiana who'd had the same Coffee machine for 30 years. She said she cleaned it with vinegar every 5 years. Made cracking coffee.

I use half a box of wilko's Citric acid on my Delonghi. Works fine