Avoid Domestic and General, just look at their reviews on Trust Pilot.
My elderly mum was left in February,19 with no heating and hot water for about a month and Domestic and General didnt give two hoots. She had to wait 4 days before anyone turned up, even though her account was noted as vulnerable. they were there 5 minutes before they said it needed parts and would come back, I had to chase constantly because no one came back to us, and it was almost 10 days later before they turned up to repair it. And then they found something else wrong, went away again, still leaving her without heating. We cancelled the plan straight after. We've found Homeserve much better, more responsive and they have a sense of urgency about them.
Like you, I was sat on the fence about my own boiler cover for a while. We had been paying £30-odd a month boiler cover for 7 years, so we'd spent circa £2500 and we'd only ever had two calls outs. I was quite frustrated with myself as it definitely wasn't value for money because no way would two call outs have cost £2500 from a local plumber.
However...... I moved house and had a new boiler that had a 5 (ish?) year guarantee, and after that I extended the guarantee into a cover plan. We've had three call outs in two years, all of which needed parts, so this plan has (so far) been well worth the money.
So, I'd stick with a cover plan personally. It's like any insurance - you don't realise how valuable it is until you need it.
And also on a personal experience/opinion.......I prefer having cover, so I know who to ring. It's one number, and they arrange everything. Yes, I might have to wait 48 hours in they're busy . But without cover, you have to try and find tradesmen, do all the ringing around yourself, make sure you have the money to hand and deal with the whole darn lot. There is a cost of convenience, and cover is one I'm prepared to bear.