Three bathroom: megaflow, heatstore, or big combi?

Hello all,

I've had my house massively extended, increasing it from one bathroom to three! So far I have kept my existing small combi boiler, but as expected it's not providing anywhere near enough hot water for three bathrooms. I've spoken to a plumber who has suggested three options and suggests they'd all be much of a muchness in terms of their efficacy:

• Add a megaflow to my existing combi boiler. The disadvantage is that I have two separate bits of kit (boiler and Megaflow) to maintain or replace when they're worn out, but it would "only" cost about £1200 compared to around double that for the other two options.
• Swap my boiler for one with an integrated hot water tank (I think it's known as "heatstore"), such as an Ideal Istor.
• Swap my boiler for a high-capacity combi boiler (such as a Worcester Greenstar Highflow 550CDi) -- my plumber says I have very high mains water pressure so this would work.

Does anyone have any thoughts as to which would be the most sensible? I have to say I'm finding it hard to get my head around how a megaflow actually works -- is it seamlessly integrated with the boiler system? Ease of use (no faffing around turning different apparatus on/off, or timing hot water etc.) is important to me.


Very grateful for any help.