Unless you fit the Costello mould (middle class Oxbridge graduate, signs off internal emails with "Best wishes") , you will struggle to fit in and enjoy working at Costello. Although the company has a great external reputation, I suspect that the Best Companies awards which the company has received for a number of years is not truly deserved - I rated the company really highly on this survey only so that it would look good for my future employers, not because I actually enjoyed working there. I know that many of my colleagues did the same thing. The company culture feels horribly insincere.
As for salary (a faux pas topic of discussion in the company), I was unhappy with my reward package given that I worked very hard. As an analyst you can expect to be taken advantage of. Analysts receive very, very little reward for their hard work (ie no bonus) - they only get an occasional (50-100 GBP) gift voucher despite doing most of the work. Salary progression is also not great. Analyst starting salary is 30,000 and it increases by roughly 1,000 each year (no bonus). Senior analyst salary is 38,000 and you can get up to two bonuses each year (roughly 3,000 each). Consultant salary is 45,000 and, although I do not know the size of the consultant bonus, I assume it is also two bonuses annually of at least 3,000.
Finally, regarding the senior management team, this team are all members of staff who are very friendly with one another as they have built the company together. Although I admire how much they have grown the company, because they are such a tight-knit management team, they do talk behind everyone's back - this potentially intensifies the uncomfortable culture of insincerity.