FRopm what you said in last paragraph, if you mean you boiught a condensing system boiler you need a dhw cylinder or you won't have any hot water.
Prices of that sort of job vary upwards of 1800 to 2,600 in this area but that would include supplying everything.
When you supply the boiler you put peoples' noses out of joint because they like to use the stuff they would attach their reputation to, and because they will always get a better deal than you, and service backup for them will always be a lot better than for you who nobody knows at the outlet where you bought it. If you got it mail order good luck to you.
We build up a good raport with out local merchants get a good price and the slightest thing wrong with anything they takle it straight back and give us a branbd spanking new one no questions asked.
OTOH it's always nice when you can leave responsibility for all that with the customer.
I had a lady bought her own rads at bbq, if anyone is clueless how to handle rads it';s the fork lifte drivers at bbq. They were absolutely mangled, I had to fit them as was out of time for the job and couldn't take them back for her. She supplied them it was her problem. I was round there the other day, she staill has fork lift bullied radiators all over the house.
Another customer bought their own two bathroom suites, when I went to the job to start work I discovered one toilet cistern cracked and an important part of walk in shower missing.
Leave it to those who know how to handle such equipment to supply iut for you.
Rant over, I have to go fit a boiler.