My plan is to keep this for 2 years and then trade it in for something a bit bigger i.e. the DS5 or DS7.
You realise this will never happen right?
By 'lowering the monthly payments' the Sales Manager has just upped how much negative equity you'll be in at the end of the term (assuming the length didn't change).
All you are doing is having a new car for 2 years and then instead of being in £2k of negative equity it'll be £5k and you'll be back at the dealer lubing up to accept a rubbish overpriced car because you "had no other option".
Borrow £8k, pay off the negative equity and buy a £5k car. Pay that £8k loan off over 2 years and at the end of 2 years you'll owe nothing and have a car worth £3k. This could then be used as a deposit towards a newer / better car (or you could just keep the £3k car and be £315 better off each month).
A 7.7k loan over 2 years is £331 @ 3.3% so I might as well keep the DS4 and then change it after the 2 years? What would I be gaining apart from a higher monthly payment a second hand car?
At the end of 2 years the 2 options result in:
1) DS4 - massive negative equity, unable to pay it off so stuck getting another rubbish finance deal.
2) Positive equity, no debt and a working car that you actually own.
The dealer could knock another £100 pcm off your payments if you ask him nicely? You'll just be in £10k of negative equity come the end of the term instead of £5k. All he's done is moved the debt to the end of the term and screwed you in 2 years time.
Your deal is awful, I'm not recommending this but as an example if you only want to consider monthly payments:
https://www.selectcarleasing.co.uk/.../saloon/20i_prestige_4dr_auto/78734-2018.html
Jaguar XF (£35k list)
10k pa
£6501 over 24 months = £270pcm
Why are you paying £45 extra a month for a Citroen (hint - the answer is because you spiralling into more and more negative equity with each deal)?