Doing salary sacrifice car scheme, on DMP??

We are about 18mths into our 10yr DMP with SC..

Our car which we bought long before starting the DMP is about 12yrs old now and last year cost over £500 to pass the mot, with a pile of advisories..
This year most of those advisories need doing, and its gonna be another 500-600 for the mot again.
Its a 1.8 petrol so tax is heavy about £270pa, and the insurance is over £500pa, plus tyres, servicing, repairs etc.

When we worked out everything and divided by 12 we are looking at around £150pm now to keep it on the road, not including what it loses each month in depreciation too.

As we cant get credit due to our DMP, our only options seem to be

A) keep going.
sell it, and buy another banger with the money
C) to change it by saving up and sell or part-ex, but with paying the current running costs and the DMP etc we cant save a whole lot and it will take about 1-2yrs to save anything worthwhile, and by then our car will have lost half its value again along with two more mots to pass, costing who knows what.

So my company has recently started offering a salary sacrifice car scheme, and we can get a brand new small economical car all costs paid for, for around £250pm.

The plan would be to sell ours, use that money of offset some of the difference between what we pay now and will pay then, until it runs out, which should be around a year or so.

Also our current fuel bill is about £150pm as its a 1.8 petrol, the trip comp shows about 27-29mpg most of the time, the car we can get is a 1.3 diesel with book figures of around 60+mpg..
so our fuel bill will drop nearly in half and that saving of £50-75 can also offset the increase in the monthly cost...

Basically it seems our best option to get a half decent car within the next few years, its hassle free no more worrying about how much the mot or tyres will be every year etc, and we think we can afford the monthly increase with our car sale, saving on fuel and a few cutbacks...

But am wondering if it will create any problem with SC/DMP/creditors if we tell them, and do we even tell them or just keep going as we are as its technically not taking out credit, nothing is in our name and there is no credit check involved.