Lloyd Grossman are awful, just my opinion. I've tried a lot, the one I end up with is Pataks Vindaloo, sauce or paste. Look for the small qualifying details, e.g "a spicy concentrated sauce with crushed chilli and spices". Patak's do several varieties of vindaloo, and I for one steer clear of anything with cumin. Tonight I am trying their Kashmiri Masala paste, subtitled "Chilli and Garlic. Hot"
The best way to hot up your curry is with a handful of green Kenyan chillies. I buy them by the handful and freeze them.
Extra hot chilli powder such as Rajah will give you a very hot curry, but it is a dry pasty sort of heat, not the nasal burning mouth-searing experience we all strive for.
I don't expect you have any Dorset Naga chillis to hand, but these are the ultimate.