100mg DXM is not enough. 150 is a good starting dose.

Here in the US, generic pills containing pure DXM go for about $5 for 300mg. Name brands are a bit more. I dunno what UK drug prices are like compared to US, so YMMV.

Check the active ingredients list on whatever you buy (I highly recommend the pills/gelcaps over syrup). It should contain only Dextromethorphan HBr as an active ingredient.