As Martin Lewis always says, buy it when you book your holiday and definitely not worth not disclosing pre existing conditions, yes your insurance will inevitably cost more but the risks of not doing just aren’t worth while and essential to ensure it covers what you doing, DS recently went trekking in Madeira, not an issue you’d think , but as much of it is in mountains above 3000m, his normal insurance just didn’t cover it, cost about £16 for one that included helicopter search if needed, really not worth the risk