Thank you all for the feedback.
I ended getting the 2.5tog ErgoPouch sleep suit after a lot of consideration, mainly due to the cost (£55.99 seemed a lot to begin with)... but my little one absolutely loves it and I'd say it's well worth the money as it replaces the duvet completely and could easily be taken away on holidays etc (we've taken it to the grandparents for a weekend trip and it works out perfectly, no need for duvets there anymore!).
As you can see on the night photo - the duvet is still far away from her but now she's no longer cold as she's snug in the ErgoPouch sleeping bag!
The material is really, really lovely and soft. The inside arms in particular almost feel like fleece but definitely aren't making her sweat in any way. The zips and buttons are also done really well, there is no scratching under the chin, there's nothing that seems to make her feel uncomfortable about it! In fact, she thought she should wear it (as some sort of onesie) throughout the day as well and I struggle getting her out of it in the mornings!
We love that the feet have the grey anti-slipping part on them so she's safe walking in them. I think the only problem there could be if the child is still a lot smaller than the bag and might stumble with the feet, in which case I wouldn't let them go up and down stairs.
We haven't used it as a 'traditional' sleeping bag but converted with the legs separated and this has proven very successful here in our household as it allows for a lot more freedom, be this in terms of walking as well as being able to stretch out during the night. I'd definitely recommend it for a toddler!
Having the long arms is also great as there really is no need for a blanket or duvet, happy days! I also find it washes well and can see it lasting a long time even when being used every night. So all in all, I think they're worth it and John Lewis has them in three lovely colours to choose from as well. :-)