You are surer of germination if you wait util the soil temperature has reached 5degC - Wilkinsons do a very cheap soil thermometer, a wise investment - and this has been a late spring with soil slow to warm up. Planting parsnip at any time to the end of April usually gives good results - even sowing them in early May isn't a disaster. Leave it too late and the soil may become too dry, and the parsnips not have time develop long roots before the summer sets in. However, if we get a wet cool spring, and you haven't had any success yet, it's worth a try.
Parsnip seed should be used fresh, so there isn't much to be gained by saving it for another season. Despite this, we've seen good results from one-year-old seed, so if you've had a failure, and have some of last year's seed left, pop it in.
Parsnips, like sprouts, only develop their full flavour after a frost, so don't try them as a summer vegetable, leave them in the ground to sweeten up.