It's worth what you can get for it. Will you just be selling a list of customers or will it be a full 'turn-key' business, i.e. absolutely everything included, all the buyer has to do is go out and work. From a novice's point of view the latter is worth far more.
Would include:
Training - Fitted van with signwriting to new owner's requirement (for the sake of a couple of hundred pounds, this is well worth it) - all equipment (Ro/Di, pumps, poles, resin, storage IBC if separate purification system) - full kit of traditional tools plus personal introduction to all the customers and full instruction in record keeping plus guarantee of "probationary" period offering help and advice while the buyer finds his feet.
Offered like this it would be much more attractive and worth many times more than you might get for a 'round'.