Gangas in Tias is brilliant for all those kinds of sauces. Wouldn't be surprised to hear that they have salad cream too Elle, so might be worth a detour some day you're passing by.
Oh we can get Salad Cream in Ropers anyway, B - it's just really expensive here.
MrZandos - best chips on the island are not actually in a chip shop... but in the Cutty Sark in Costa Teguise. Bloody lovely!