SIPP - contribution carry forward rules

[Twintowers]

My first post - please be gentle lovely Lemons.
We are looking to open a SIPP for Mrs T and have funds to potentially top this up to make the most of backdated contributions for the last three years. Im not sure that I'm understanding the rules around this correctly though.
She has been working part time through those years (earning about 12k pa) in employed and s/e work. She has been in a works pension scheme all that time so ticks that box. Am I right that she can backdate using a total of £36k as total earnings for the period. And If she puts that amount into a SIPP it would be topped up to £45k with the tax relief. She's 55 and has not drawndown from a pension.elsewhere. Most of the guidance I've looked at seems to suggest this is correct but elsewhere there are references to "paying in 40k in the current year to qualify for backdating". Hmmm have I got it right?
In addition to a lump sum into the new SIPP she will be transferring in three small works pension pots. I think I am correct that these will have no effect on the lump sum amount or annual 40k limit.
And finally, once the lump sum goes into the SIPP, does it have to be invested immediately to capture the tax relief or can some sit in the SIPP as cash so she can drip feed it into a fund (or whatever) over the next year or two?
Many thanks if you can help with the points above or feel I may be missing something altogether!
Mr T

[genou]

Am I right that she can backdate using a total of £36k as total earnings for the period. And If she puts that amount into a SIPP it would be topped up to £45k with the tax relief. She's 55 and has not drawndown from a pension.elsewhere. ... Hmmm have I got it right?
In addition to a lump sum into the new SIPP she will be transferring in three small works pension pots. I think I am correct that these will have no effect on the lump sum amount or annual 40k limit.
And finally, once the lump sum goes into the SIPP, does it have to be invested immediately to capture the tax relief or can some sit in the SIPP as cash so she can drip feed it into a fund (or whatever) over the next year or two?
Many thanks if you can help with the points above or feel I may be missing something altogether!
Mr T

Doesn't work that way I'm afraid. You can only get relief for contributions that you can match to Net Relevant Earnings in the year of contribution. So the limit of 40k can be extended by brought forward allowances only if your NRE exceed 40k. If I read what you are saying about your wife's income correctly, she can't bring any relief forward.

The contributions can sit in cash should you wish - probably earning zero return depending on your SIPP provider.