as you appear to understand, SDLTY is based on the consideration paid for the property itself, not including any chattels it may contain.
What you should be VERY wary of is a claims management company whose business model is to allege that the value of the chattels is "high" and therefore the inclusive price paid has over inflated the SDLT due.
When completing the SDLT return the value of chattels has to be declared. HMRC computers are not going to miss high figures and may cause the return to be pulled for review by a human. Will the claims management company be around when occurs, or will they have taken their fee cut and gone away?
HMRC's stance on the valuation chattels is clear cut, and if you were to purchase them separately without being linked to a property, your start point in a price negotiation would reflect HMRC's approach, not the so called claims management company's "cost price".
In real terms: would you really pay original cost price for a 20 year old set of curtains or a 5 year old freestanding fridge?
Of course you would not!
You would pay current second hand value (and most people still over estimate that as few have genuine antiques with a rising second hand value!)
Have you any experience of what a "house clearance" company would offer when clearing the entire contents of a deceased person's house?
I know of occasions where the relatives decision to retain the white goods resulted in them being charged to dispose of the remaining furniture as it was considered "brown furniture" and thus worthless. In the case of my uncle, inclusion of the white goods resulted in receipt of £350 to purchase the entire household contents: clothing, furniture, carpets, white goods, china, cutlery etc etc.
HMRC will apply that logic, assess you have made an "incorrect" SDLT return, and make you pay the extra tax due (+interest). Bet the claims management company will not refund their share of the now due tax!)
SDLTM04010 - Scope: How much is chargeable: fixtures and fittings - HMRC internal manual - GOV.UK