When I installed my central heating I unwittingly made the vent pipe to the header tank in 15mm instead of 22mm.  This, as you will all know, results in the water coming out of the overflow if the pump is set to too high a speed.  I currently run with the pump on its lowest speed setting which is OK but makes the system slow to heat up.  I am loathe to replace the whole pipe run with 22mm as at least half of it is behind a recently tiled box.  My question is this:  Can I just replace the pipework in the loft with 22m or 28mm?  Will this solve the problem (or at least help)?  The pipework in the loft consists of about 5ft horizontal run and 4ft vertical.  The amount of pipe above the header tank water level is around 2ft.
Thanks
Jim

In other words it might not be the size of pipe that's the problem.

What the-scruff needs is a photo and or drawings showing where the feed and vent connect to the system in relation to the pump.

Can't get my photo to upload so I'll have to describe the relevant parts of the system.  Apart from the vent pipe and feed pipe the whole system is in 22mm.  Between the boiler flow pipe and the pump is a feed from the header tank.  The pump direction is from the flow pipe towards the diverter valve and off to the rads/tank etc.  The vent/overflow pipe is tee'd off the return pipe and this is the last return tee before arriving back at the boiler return inlet.  Distance from vent tee to boiler about 7ft. Distance from feed pipe to boiler about 7ft.  Distance from feed pipe to pump about 1 ft.  Total run of the 15mm vent pipe is: 12" vertical, 10 ft horizontal, 8 ft vertical, 4ft horizontal and 3ft vertical to the overflow bend.  Height of the overflow bend is about 18" about header tank water level.  Of this the last 7 ft is accessible.  The system was designed around the info which came with the boiler and, except for the vent pipe diameter, follows the standard layout which can be found in many places.  The pump has just been changed (by a real plumber!) and the system drained, refilled and checked for correct operation.  There are no blockages.  The system works well at the slow pump speed, albeit a little slow to warm the rads.  The system bleeds easily.  If nobody can make sense of my original question which was if I replace the pipework leading to the header tank with 22m or 28 mm, will this solve or improve the situation, I will leave it alone and put up with a slightly inefficient heating system.  Thanks to all who have responded.  I thought this may be a well known plumbing problem with well documented solutions.  Looks like I'm wrong.
Jim

Google found a drawing..

The plumber should know how it goes, otherwise refer to my post or CWatters pic.

Anything else is wrong.

Yes you can have a combined feed and vet, not the best plan though

CWatters first pic is how it should be done, and if your plumber doesn't agree, get another one that knows what he's doing.