Short answer: if the sofa bed will be slept on a handful of nights a year, a standard mechanism with a decent mattress is fine. If it is going to be slept on most weekends, or every night in a spare room that doubles as an office, buy one built as a bed first and a sofa second. The thing that separates a good sofa bed from a bad one is almost never the sofa part. It is what is underneath the cushions.

Why most sofa beds disappoint

Everyone has slept on a bad one. The thin mattress that lets you feel every bar, the dip in the middle, the clank when it opens. The cheapest sofa beds use a thin foam pad over a sprung mesh, which is comfortable for about one night. After that the mesh shows through and the guest wakes up with a sore back and a story to tell.

So the first question is an honest one: how many nights a year will this actually be a bed? Christmas with the in-laws is three or four nights. A grown-up child home from university every other weekend is forty. A spare room that someone sleeps in every night is a bed with a sofa attached, and should be bought as one.

The three levels, in plain English

Standard mechanism. A three-fold metal action with a sprung base and a proper mattress on top. It folds up small, so the sofa looks and sits like a normal sofa when closed, and it is the right choice for occasional guests. In our own range the Lion, Breeze and Oasis all come as a standard version.

Deluxe mechanism. A two-fold action on beech sprung slats instead of a sprung base. Fewer folds means a flatter sleeping surface with no ridge across the middle, and timber slats give more support than mesh. This is the one to pick if guests stay regularly, or if the people sleeping on it are older or have back trouble. Same three sofas, same sizes, a bit more money.

Everyday sofa beds. These are built the other way round. Our Italian everyday sofa beds hide a real mattress, 14cm to 18cm thick, on a fold-out frame, with a removable cover you can wash. They are designed to be slept on night after night, and the sleeping surface is a true single, double or king width rather than a cramped compromise. If you are short of bedrooms and the living room or box room has to do two jobs, this is the category to look at.

Check the sleeping size, not the seating size

A "3 seater sofa bed" tells you how many people sit on it. It tells you nothing about who can sleep on it. Always ask for the sleeping surface in centimetres. As a guide, a small 2 seater usually opens to about a 3ft single, a large 2 seater to a 4ft small double, and a 3 seater to a 4ft 6in double. On the everyday range the sleeping surfaces run from 120cm wide on a 2 seater up to 160cm on a 4 seater, with a 192cm length, which is a proper king-width bed. If two adults will be sharing it over Christmas, a single-width sofa bed is going to cause an argument.

Measure the room with the bed open

This is the step people skip. A sofa bed roughly doubles its depth when opened. The everyday models open to just over two metres from the back of the sofa to the foot of the bed, and the standard range is similar. Stand in the room, measure from the wall the sofa will sit against, and mark two metres plus walking space. Check what that line crosses. A coffee table can move. A radiator, a door swing or a fireplace cannot. If the open bed blocks the door into the room, you have bought a sofa, not a sofa bed.

Fabric, pets and the washing machine

A sofa bed in a family home takes the same punishment as any sofa, plus the extra wear of being opened and closed. Choose a fabric for the sofa part the same way you would for any sofa; our sofa fabric guide covers velvet, chenille, linen-look and leather and which survives kids and dogs. For the bed part, a removable washable mattress cover matters more than people expect. Guests spill tea. Children have accidents. Being able to take the cover off and put it through the wash is the difference between a sofa bed that lasts and one that quietly starts to smell.

Will it arrive before Christmas?

Here is the honest bit. Our sofa beds are made to order in your choice of fabric, and the standard and deluxe ranges are quoted at 8 to 10 weeks. If you are reading this in the second half of October, that lands somewhere around the week of Christmas itself, which is cutting it fine. Lead times move about, so ring us or come in and we will tell you the current date for the sofa you want rather than guessing. If you need something sooner, say so and we will tell you what is realistic. We would rather lose a sale than have you sitting on the floor on Christmas Eve waiting for a delivery.

Online orders come with two-man delivery for £80, and if the budget is tight before Christmas, 0% finance is available on sofa beds too.

Try it as a bed, not just a sofa

Photos online all look the same. The only way to know whether a sofa bed is comfortable is to open it and lie on it. We have sofa beds on display in Clydebank that you can fold out and try, and we will tell you plainly which ones we would put a guest on for three nights and which ones we would sleep on ourselves.

Browse the full range on our sofa beds page, or come in and open a few. Bring the room measurements and the number of nights a year, and we will point you to the right one.

Paul James Home, inside the Co-op Department Store, Clydebank. Monday to Saturday 9 to 5. 0141 266 0400.