Good luck, at 23 I still couldn't get anything insured under £800, so bought an MG midget, roof's are overated anyway 
Classic insurance insn't an option untill your 21, and unless you can convince them that a '89 sierra estate is a classic you'll be looking at anglia's and travelers etc for classic estates, which barely have more space than a modern fiesta/astra. Classic car insurance companies are getting wise to the idea that kids can buy a banger and insure cheep as a 'classic'. When I got mine it felt more like applying for a job at an interview than applying for car insurance!
Also, ignore moneysupermarket, comparethemarket, gocompare.com, etc, go to a broker (there migth be a few on your high street) and get them to search for quotes, because it saved me about £150 which was 25% of my bill. The websites just enter your details into every other online insurance website and sumarise the results for you, a broker might actualy know a cheeper place, or have negotiated discounts with verious companies.