You should cancel the phone call request to avoid another expert calling.
Only gains made since 31 March 1982 are taxable so you would need to know what the mobile home was worth as at 31 March 1982 to get a truly accurate gain figure. In addition, as you inherited your late friend's half, that half will have a cost equal to its value when your friend passed away so your cost for CGT purposes will be greater then £10,000. The costs of improvements (not general maintenance) can be deducted from the gain. However I will use a cost £10,000 for the purposes of this answer.
From March 1982 to the present is about 34 years. You appear to have lived in the home until about 1989. The gain for the period that it was your main home will be exempt from CGT as will the last 18 months of ownership regardless, a total of 8.5 years. That covers £7,625 of the gain (£30,500 / 34 years x 8.5 years). That leaves £22,875 (£30,500 / 34 years x 25.5 years). The first £11,100 of that will be tax free so you are left with a net taxable gain of £11,775.
There are two rates of CGT for residential property, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your income in 2016/17. Assuming that your income is no more than £31,225 per annum pre-tax, you will pay CGT at 18% on £11,775 (£2,119.50). If all the net taxable gain was taxable at 28%, the CGT would be £3,297.00). Ultimately, your CGT liability should be lower than either £2,119.50 or £3,297.00 since your cost will be higher than £10,000 as I described above.
I hope this helps but let me know if you have any further questions.