A word of caution , and not aimed at any actual dive retailer..... Look out for short test dates - on new cylinders check the date of manufacture - as these are often sold cheap, or you could get stung for a test very soon after you buy.
I'm toying with the idea of getting rid of mine at the moment. Don' t really need the extra air and looking at either swapping to, or buying an additional, 12 litre.
I'll prob see you in Lincoln tonight and we'll have a chat. You may be able to persuade me to get rid.
If I'm feeling responsible, You may consider sticking to a 12 litre. Your air consumption is likely to come down dramatically in the first year of diving. If you then progress to twins, you can just get another one and manifold them up (or not for the indie fans out there).