in case it's of help to others, this is the state of play so far from the first two to respond, which were Paystream and Nasa.

1. Both will pay in to a HL SIPP. Woot.

2. Both do carry-forward. Double woot.

They do not check contribution limits in any way, it's the responsibility of the worker to contact their pension provider and make sure the contributions are diverted appropriately (various years). Which is fine, it's as I hoped.

My question regarding overpaying...

Mechanics of Salary Sacrifice.
Paystream said the way it works is that HMRC only allow contribution variations when there is a change of circumstances. That can include being at the end of a contract and it can also be varied at the end of each tax year. However...

One can simply fund to the maximum until the available pension contributions are used up and then cease contributions for the year, then re-start the next year. There are no issues with doing that apparently.

There is a small question he couldn't answer over what happens if you get near to using up all your cont. limit and the next payment takes you over... he's gone away to find out about that.

Nasa solution. So these two seem very closely matched in terms of the features, but one other point worth mentioning is that Nasa's solution is a "hybrid" where you can use your Ltd co and be inside and outside IR35 at different times. I'll need to delve into that more closely.

I'll post more when/if I hear from the others - I've yet to compare costs etc but I suspect they'll be much of a muchness.

QUESTION: as I read the other threads I noticed a few disparaging comments re Nasa - but nothing concrete as to why. If anyone can share reasons not to go that route I'd be interested/grateful.