You don't necessarily need an accountant. You can do all of your tax returns online and you can use your annual CT600 tax return as your return for Companies House. The annual return is just a few pages and can be completed by anyone who can read the explanatory notes. Hiring staff is only marginally more complex and HMRC provide a suite of software to help you manage payroll, income tax and national insurance. The only real gotcha is remembering to register for VAT if your sales within the EU exceed £73,000 in any 12-month period - at that point you may wish to consult an accountant to get your VAT accounting straight, as mistakes with VAT can get expensive quickly.
As a web app startup your tax affairs are likely to be as simple as they get and I'd suggest contacting your local HMRC office for advice - I think you'll be pleasantly surprised at how helpful they are and how straightforward tax is for small companies. They run regular training days at local tax offices for new companies and employers which are well worth attending even if you plan on using an accountant.
They can also provide references which can expedite things like opening bank accounts or getting travel visas.
IMO Unless you really can't afford it, paying the £1000/year for an accountant so you have one less set of things to worry about as a startup founder is really worth it.
They offer cashback if you later open a business bank account with Barclays, so the company formation is actually free (you can even get back ~£30 back on top if you pick the cheapest formation plan).
As for accountants I've been using http://www.crunch.co.uk/
For £70 a month they give you:
* web-based accounting software
* dedicated account manager
* dedicated accountant
* sort out your taxes at end of tax year
* registered office service
* extras like help you with personal taxes for extra fee
Highly recommend both companies.
Unfortunately CSV statements from my bank (Barclays) aren't compatible with their importer. Hopefully they'll add support for more statement formats.
Experience: UK resident who set his own Limited company up
http://ycombinator.com/atyc.html
Getting your incorporation wrong can be a deal killer.