My thai wife has an indefinite leave visa in a previous passport and we have regularly travelled in and out of the UK with her carrying the old passport with the visa and her new passport.

She has come across reports/rumours of this no longer being valid, I have had contradictory advice from UK officials and it seems that the final view was 'its down to imigration control' on the day we travel.

Has anybody else experienced problems or aware of any changes for Thais?

BKK

I'm assuming you've read through this, https://www.gov.uk/transfer-visa

ILR is indefinite and doesn't lapse because the passport the vignette is stuck into has expired.

As it says on the page linked to by theoldgit:

You don’t have to transfer your visa - you can carry both your old and new passports when travelling to or from the UK instead.

However, if your wife needs to prove her UK residency status for some other reason, for example to a new employer to show she can legally work in the UK, she may have problems as they may not be aware of the rules.

So maybe obtaining a BRP is worth considering; despite the cost.

Alternatively, why not apply for British citizenship when she meets the requirements, if she doesn't already?

She can then forget about needing UK visas or residence permits of any sort. Also, should you ever move back to Thailand her ILR will lapse if she spends a continuous period of 2 years or more out of the UK. Even if she returns regularly to the UK for visits, immigration could cancel her ILR if they have reason to believe she is no longer a UK resident.

Once granted, British citizenship cannot lapse and can only be revoked if obtained by fraud or the naturalised citizen is convicted of a serious criminal offence.

Both the UK and Thailand allow dual nationality, so naturalising as British will have absolutely no effect on her Thai citizenship or rights as a Thai citizen.