Condensing oil boiler - size and recommendations

Hi,
I am planning to install an oil condensing boiler and I need to decide which model and size to use. Grant with Riello burner has been recommended. I will be putting it in utility room and I see Grant have 3 sizes in their condensing indoor range, 15-26kW, 26-36kW, 36-46kW. House is 2100 sq.ft bungalow with underfloor heating throughout. There is also about 800 sq.ft in attic space (insulated) which will have a few rads but will be maintained at much lower temp than downstairs. There is also about 7 sq.m in solar panels but these are only used for DHW.

Can anyone advise on how to calculate the size of boiler I would need ? Also any recommendations for other manufacturers or models would be welcome.

I have had a grant installed to work with solo rads and barlo rountops. Its a bungalow 2000sq foot. I work in BTU's and got a boiler for 120btu's I think. But go here and download this piece of software and you can calculate for each room and then get a final figure and you can switch between BTU's and KW's. I did a lot of reading and finally settled on the grant.

A good plumber will use a wheel device to size house exactly. The figure I got from the piece of software was very close to his.

8888.w.quinn-radiators.co.uk/downloads_heatloss.php

good luck