Executive summary – you need to get some proper advice
The employer saves their national insurance, something like 10% or 14% – so if they do the same amount X in salary or pension they are benefitting.
EDIT: just seen above that the employer there pays the extra – what is yours proposing ?
As noted above the employer benefits as redundancy etc is calculated on salary, also is it a final salary scheme – if so that could work against you
Why would you not just make your own AVC (Additional Voluntary Contribution) – ie put these amounts into your pension at your discretion at the end of each year based on whether you think you can afford it ? There is a slight timing issue here as you have to claim the tax back later if you do a lump sum, you can do regular avc’s monthly.
I think the level of your salary wrt national insurance rates is important.