I've spent the last two days working on my Worcester 280 combi. The symptoms were hot/cold DHW and occasional tripping out. I replaced the secondary heat exchanger for a plate exchanger (a Worcester upgrade) and put a new expansion vessel in. I fired it up last night and CH was good but had problems with DHW. Within a minute of running the hot water the pressure rose rapidly and the boiler tripped out. I picked it up again today. The diverter valve had a slight seep so I decided to whip it out and check it over. The bottom retaining clip had rusted in so cut a long story short I ended up cutting it out! Picked up a new DV (£100!) and tried to fit it. Bottom retaining clip wouldn't fit . 5 hours later it turned out that the new DV had a faulty plastic centre joint ( a manufacturing fault i'm sure) so I swapped it for the old plastic joint and hey presto...fitted in seconds!!All back together and crossed fingers, working OK! I see why the 'heating engineer' said to replace the combi now but I rarely let anything beat me!!!