Hi Peter - welcome to the forum! This is an area I am unfamiliar with - but hang on a BR expert will be along shortly. If you are thinking of going into an IVA there is a sister site (link on top left hand column - IVA Help) that will be able to support you into and through an IVA.
This is quite a grey area and varies significantly from lender to lender. The key is to remember that an IVA is classified as personal insolvency so it would have a negative impact .... exactly how negative would depend on the lender.
Sorry the answer is a bit "wooly", but it's a very grey area!
the secured part would be excluded from the IVA and you would maintain your mortgage payments the unsecured debt would form part of your IVA and the mortgage company would therefore become a voting creditor in your IVA.