I'm afraid I don't have that exalted status. I did try it once but nearly drowned, I was only saved when some bloke in a long flowing white robe and rather stringy beard came along and pulled me out. Don't know what he had on his feet, but he didn't seem to be having the same sinking problem as me.

Anyway, back to your door.

It's a common repair. Slotting the "block" in you'll hear described as "scarfing in a repair piece."

Look carefully at the length of wood that the door closes against all around the sides and top, that's your rebate (nothing to do with council tax). If it's obviously a separate piece "planted on" then you prise that off and discard it 'cos it's probably as rotten as the casing, its removal also makes the next bit easier.

You really need to cut out the rotten piece using a 45^o cut going upwards from its lowest point on the door side of the frame to its highest point where the frame meets the wall. That's what a chippie would do. However, unless you're prepared to have a go at that, you could just get away with making a straight cut across to remove the rotten bit. Take the same height out from BOTH sides to make it look as if you intended it to be that way!

Before doing that and leaving your door with bloody big square holes either side of it, you'll have measured up the size of the frame and either gone down to B&Q for some wood of the same section or gone to a local builder's merchant, flashed your baby blues at them and got them to cut a piece to size for you. (Tight fitting t-shirt and mini-skirt will probably get them offering to come round and do the job for you at the weekend! An optional alternative.)

Without the rebate on, the frame will be just a rectangle. Hold the new wood against either the cut away frame or the bit you've just cut out and mark the new wood so that it will fit the hole when cut. Do this whether you've managed the 45^o cut or not. Cut the new repair piece to size.

The best and easiest way to fix that repair piece (especially if you haven't angled it) is to glue it in place. If you have angled it then you still need to glue it, but will have the advantage of being able to screw through the frame into the repair piece to 'clamp' it in place until it's all set.

In this situation, you'll probably be best off using an expanding foam (because it will expand to fill any unevenness in the cut and fit), but go easy with it and wear rubber gloves to keep it off your hands because it goes black and won't come off for a few days, until you start to moult! It's a bit like squirty cream and comes in either a high or low expansion version. Use the low expansion type if you can, but the other type will be fine if you go easy because it'll go EVERYWHERE.

If you've cut the pieces square then measure carefully across the inside width of the doorway and cut a length of wood to this exact size - that's your spacer. Once you've 'glued' each side in place (having put the foam on the top surface where it meets the original frame, and on the side that goes against the brick) then drop this spacer into position to hold each piece in place until the foam adhesive goes off, which should be in about an hour, but before you leave it to its own devices, make sure it's hard to the touch, if it bends then leave it longer.

Trim the excess foam off, going just a fraction into the joint and sand it down. Put a smear of wood filler into that recess you made when you trimmed off the excess. Sand that when cured, again about an hour.

To replace the discarded rebate you'll have taken the discarded bit with you to the builder's merchants and got a couple of lengths of the same section, either off the shelf or cut to size for you. Hopefully, they won't have been mitred (cut at an angle) at the top, but either way you can either cut the angle to fit and THEN cut to length or just cut to length and nail in place.

Paint. Job done.

If that rebate is NOT planted on then just cut across to remove it along with the rotten section of frame, fix the new repair piece in place and then "plant on" the missing bit of rebate.

Is it just on the outside? One possibility might be to insert identical blocks both sides. Leave them proud so they look like pillaster blocks.