Worcester are a highly regarded boiler manufacturer, but suffer from being over complicated.
I researched this very problem only 12 months ago and came up with a company that is not very well known in the UK, but has an exceptional (in my view) design that drastically limits the number of moving parts. They're called Intergas. A Dutch company that has been in existence for years and are well regarded in Holland.
You can find a suitable boiler here, but be aware, I would recommend you use a factory approved installer, as most don't know what they're doing with these boilers. The factory approved guys, have all been required to attend factory training in the company's products and are fully competent to install and maintain the products. Interestingly, the gas engineer I used for installation, had this very make himself and the company I sourced it from, While having the choice of just about every manufacturer you can think of, also had one heating their warehouse.
In truth, I didn't find them any more expensive than any other and in all probability, cheaper than the Worcester equivalent.
The link I've given you is for a open vent system, which from your description, seems to be what you've got and want to keep. But all the other options are available.
I've no doubt, that unless you've had a "power flush" recently, any good installer will recommend you have one and looking at the gunk that came out, rightly so. You hardly want to clog up a new boiler from day one!