As other's have indicated, there's no IHT payable anyway. The basic threshold is £325,000. Where someone has inherited everything from their spouse, that person also inherits their IHT allowance, taking the threshold to £650,000.
If you were to work for your mother on a self-employed basis you'd need to be able to show that you weren't tied solely to that one job in order for HMRC to accept that you were self-employed.
If your mother was to employ you on a PAYE basis, your duties could include those of her bookkeeper, meaning that you'd be responsible (on her behalf) for ensuring that all of the PAYE paperwork was correctly submitted to HMRC. (If you're to receive £20k p.a. it would actually cost your mother rather more than that as she'd have to pay the employer's contributions in regard to National Insurance. She'd also have to offer you a workplace pension but you could decline that offer).