Car allowance offsetting against company car tax.....

Grumpy_chap

So, the information from the lease company is accurate about the costs per month.  Say it is £300 gross, but £240 as a deduction from nett salary. (Or a lesser nett reduction for higher rate tax payer.)
The salary sacrifice company / lease company simply won't know how you salary package is built up, and nor should they.

What seems incorrect is the way the employer is saying this is taken from salary.

Many people earn salary £A with no perks, so total package £A.  The £300 is simply deducted from this basic pay £A.

You have salary £B plus £300 car allowance making total package £C. 
The £300 needs to be deducted from £C.
Of course, they could deduct the £300 from £B but then still give you the £300 car allowance.
Or pay £B in full and simply reduce the car allowance to nil.

What would be absurd, and must be incorrect, is the suggestion that you will receive £B less £300 for the lease car and also no longer receive the car allowance.  That makes your gross contribution to a £300 lease at £600.
This is the part where I suspect the misunderstanding / incorrect information is. 
This is where you need to request proper and accurate information rather than accept the apparently off-the-cuff answer from a junior Line Manager or Administrator.

I am quite sure there may be more senior people with larger car allowances (£400?  £500?) that would still choose a £300 lease car through the salary sacrifice.  Is the employer truly saying they would see deductions from salary of (£400 + £300) or (£500 + £300)?

Sadly, it is not uncommon for junior manager to not know but think they are correct but also not care and see any challenge as an affront to their seniority and authority (hence just reducing their credibility even further).  

Can you seek confirmation from appropriate senior manager within HR or payroll on exactly how the deductions will be administered?

ianb1200

You have salary £B plus £300 car allowance making total package £C.  
The £300 needs to be deducted from £C.
Of course, they could deduct the £300 from £B but then still give you the £300 car allowance.
Or pay £B in full and simply reduce the car allowance to nil. 

Agreed - but it appears to be the case. This is the extract from the scheme details - it's not salary sacrifice.

Salary Deduction scheme means you pay a fixed amount from your pay after stoppages. The cost of the car is reduced by a business mileage allowance based on your estimated annual mileage and you will be paid fuel for your business journeys monthly on submission of a mileage return.

Car benefit tax is applicable to the Salary Deduction method although the full car benefit does not apply as you can offset the charge for the car against the tax.

Apart from the "free" insurance etc and maybe the zero deposit I really can't make it out. The comment about junior / senior / HR colleagues isn't really relevant as I'm dealing with the lease Co. Nobody in the scheme can make it out and there's not that many it it!