What's wrong with my B2L plan?

[PhiltheGreek]

Basically, yeah. It wouldn't be different to any other personal pension contributions. Be a bit careful about the limits (remember personal pension contributions are not only limited by the annual/lifetime caps, but also your "net relevant earnings", which importantly for small biz owners excludes dividends). Otherwise, if you put in £800 (of your own, after tax money), the pension scheme reclaims £200 from HMRC leading to £1,000 in the pension. You then declare on your personal tax return a £1,000 personal pension contribution, which extends your basic rate band, meaning a bit more of your income is taxed at basic rate rather than higher rate. Ie the pension scheme gets the basic rate tax relief for the contribution, you then get the higher rate relief.

Isn't rental income classed as 'non earned' income and therefore not available for SIPP tax relief?

[WordIsBond]

Isn't rental income classed as 'non earned' income and therefore not available for SIPP tax relief?

Good point but presumably OP is paying himself a salary from his LtdCo and that's going to be more than the rental income for the property he's described.